-----------------------------------------------------------------------------
This file contains a concatenation of the PCRE2 man pages, converted to plain
text format for ease of searching with a text editor, or for use on systems
that do not have a man page processor. The small individual files that give
synopses of each function in the library have not been included. Neither has
the pcre2demo program. There are separate text files for the pcre2grep and
pcre2test commands.
-----------------------------------------------------------------------------


PCRE2(3)                             PCRE2                            PCRE2(3)


NAME
       pcre2 - Perl-compatible regular expressions (revised API)


INTRODUCTION
       PCRE2 is the name used for a revised API for the PCRE library, which is
       a set of functions, written in C, that implement regular expression
       pattern matching using the same syntax and semantics as Perl, with just
       a few differences. After nearly two decades, the limitations of the
       original API were making development increasingly difficult. The new
       API is more extensible, and it was simplified by abolishing the
       separate "study" optimizing function; in PCRE2, patterns are
       automatically optimized where possible. Since forking from PCRE1, the
       code has been extensively refactored and new features introduced. The
       old library is now obsolete and is no longer maintained.

       As well as Perl-style regular expression patterns, some features that
       appeared in Python and the original PCRE before they appeared in Perl
       are available using the Python syntax. There is also support for some
       .NET and Oniguruma syntax items, and there are options for requesting
       minor changes that give better ECMAScript (JavaScript) compatibility.

       The source code for PCRE2 can be compiled to support strings of 8-bit,
       16-bit, or 32-bit code units, which means that up to three separate
       libraries may be installed, one for each code unit size. The size of a
       code unit is not related to the bit size of the underlying hardware. In
       a 64-bit environment that also supports 32-bit applications, versions
       of PCRE2 that are compiled in both 64-bit and 32-bit modes may be
       needed.

       The original work to extend PCRE to 16-bit and 32-bit code units was
       done by Zoltan Herczeg and Christian Persch, respectively. In all three
       cases, strings can be interpreted either as one character per code
       unit, or as UTF-encoded Unicode, with support for Unicode general
       category properties. Unicode support is optional at build time (but is
       the default). However, processing strings as UTF code units must be
       enabled explicitly at run time. The version of Unicode in use can be
       discovered by running

       pcre2test -C

       The three libraries contain identical sets of functions, with names
       ending in _8, _16, or _32, respectively (for example,
       pcre2_compile_8()). However, by defining PCRE2_CODE_UNIT_WIDTH to be 8,
       16, or 32, a program that uses just one code unit width can be written
       using generic names such as pcre2_compile(), and the documentation is
       written assuming that this is the case.

       In addition to the Perl-compatible matching function, PCRE2 contains an
       alternative function that matches the same compiled patterns in a
       different way. In certain circumstances, the alternative function has
       some advantages. For a discussion of the two matching algorithms, see
       the pcre2matching page.

       Details of exactly which Perl regular expression features are and are
       not supported by PCRE2 are given in separate documents. See the
       pcre2pattern and pcre2compat pages. There is a syntax summary in the
       pcre2syntax page.

       Some features of PCRE2 can be included, excluded, or changed when the
       library is built. The pcre2_config() function makes it possible for a
       client to discover which features are available. The features
       themselves are described in the pcre2build page. Documentation about
       building PCRE2 for various operating systems can be found in the README
       and NON-AUTOTOOLS-BUILD files in the source distribution.

       The libraries contains a number of undocumented internal functions and
       data tables that are used by more than one of the exported external
       functions, but which are not intended for use by external callers.
       Their names all begin with "_pcre2", which hopefully will not provoke
       any name clashes. In some environments, it is possible to control which
       external symbols are exported when a shared library is built, and in
       these cases the undocumented symbols are not exported.


SECURITY CONSIDERATIONS
       If you are using PCRE2 in a non-UTF application that permits users to
       supply arbitrary patterns for compilation, you should be aware of a
       feature that allows users to turn on UTF support from within a pattern.
       For example, an 8-bit pattern that begins with (*UTF) turns on UTF-8
       mode, which interprets patterns and subjects as strings of UTF-8 code
       units instead of individual 8-bit characters. This causes both the
       pattern and any data against which it is matched to be checked for
       UTF-8 validity. If the data string is very long, such a check might use
       sufficiently many resources as to cause your application to lose
       performance.

       One way of guarding against this possibility is to use the
       pcre2_pattern_info() function to check the compiled pattern's options
       for PCRE2_UTF. Alternatively, you can set the PCRE2_NEVER_UTF option
       when calling pcre2_compile(). This causes a compile time error if the
       pattern contains a UTF-setting sequence.

       The use of Unicode properties for character types such as \d can also
       be enabled from within the pattern, by specifying (*UCP). This feature
       can be disallowed by setting the PCRE2_NEVER_UCP option.

       If your application is one that supports UTF, be aware that validity
       checking can take time. If the same data string is to be matched many
       times, you can use the PCRE2_NO_UTF_CHECK option for the second and
       subsequent matches to avoid running redundant checks.

       The use of the \C escape sequence in a UTF-8 or UTF-16 pattern can lead
       to problems, because it may leave the current matching point in the
       middle of a multi-code-unit character. The PCRE2_NEVER_BACKSLASH_C
       option can be used by an application to lock out the use of \C, causing
       a compile-time error if it is encountered. It is also possible to build
       PCRE2 with the use of \C permanently disabled.

       Another way that performance can be hit is by running a pattern that
       has a very large search tree against a string that will never match.
       Nested unlimited repeats in a pattern are a common example. PCRE2
       provides some protection against this: see the pcre2_set_match_limit()
       function in the pcre2api page. There is a similar function called
       pcre2_set_depth_limit() that can be used to restrict the amount of
       memory that is used.


USER DOCUMENTATION
       The user documentation for PCRE2 comprises a number of different
       sections. In the "man" format, each of these is a separate "man page".
       In the HTML format, each is a separate page, linked from the index
       page. In the plain text format, the descriptions of the pcre2grep and
       pcre2test programs are in files called pcre2grep.txt and pcre2test.txt,
       respectively. The remaining sections, except for the pcre2demo section
       (which is a program listing), and the short pages for individual
       functions, are concatenated in pcre2.txt, for ease of searching. The
       sections are as follows:

           pcre2          this document
           pcre2-config   show PCRE2 installation configuration information
           pcre2api       details of PCRE2's native C API
           pcre2build     building PCRE2
           pcre2callout   details of the pattern callout feature
           pcre2compat    discussion of Perl compatibility
           pcre2convert   details of pattern conversion functions
           pcre2demo      a demonstration C program that uses PCRE2
           pcre2grep      description of the pcre2grep command (8-bit only)
           pcre2jit       discussion of just-in-time optimization support
           pcre2limits    details of size and other limits
           pcre2matching  discussion of the two matching algorithms
           pcre2partial   details of the partial matching facility
           pcre2pattern   syntax and semantics of supported regular expression
                          patterns
           pcre2perform   discussion of performance issues
           pcre2posix     the POSIX-compatible C API for the 8-bit library
           pcre2sample    discussion of the pcre2demo program
           pcre2serialize details of pattern serialization
           pcre2syntax    quick syntax reference
           pcre2test      description of the pcre2test command
           pcre2unicode   discussion of Unicode and UTF support

       In the "man" and HTML formats, there is also a short page for each C
       library function, listing its arguments and results.


AUTHORS
       The current maintainers of PCRE2 are Nicholas Wilson and Zoltan
       Herczeg.

       PCRE2 was written by Philip Hazel, of the University Computing Service,
       Cambridge, England. Many others have also contributed.

       To contact the maintainers, please use the GitHub issues tracker or
       PCRE2 mailing list, as described at the project page:
       https://github.com/PCRE2Project/pcre2


REVISION
       Last updated: 22 February 2025
       Copyright (c) 1997-2021 University of Cambridge.


PCRE2 10.49-DEV                22 February 2025                       PCRE2(3)
------------------------------------------------------------------------------


PCRE2API(3)                          PCRE2                         PCRE2API(3)


NAME
       pcre2api - PCRE2 native API


DESCRIPTION
       #include <pcre2.h>

       PCRE2 is a new API for PCRE, starting at release 10.0. This document
       contains a description of all its native functions. See the pcre2
       document for an overview of all the PCRE2 documentation.


PCRE2 NATIVE API BASIC FUNCTIONS
       pcre2_code *pcre2_compile(PCRE2_SPTR pattern, PCRE2_SIZE patlen,
                                 uint32_t options, int *errorptr,
                                 PCRE2_SIZE *erroroffset,
                                 pcre2_compile_context *ccontext);

       void pcre2_code_free(pcre2_code *code);

       pcre2_match_data *pcre2_match_data_create(
             uint32_t oveccount, pcre2_general_context *gcontext);

       pcre2_match_data *pcre2_match_data_create_from_pattern(
             const pcre2_code *code, pcre2_general_context *gcontext);

       int pcre2_match(const pcre2_code *code, PCRE2_SPTR subject,
                       PCRE2_SIZE length, PCRE2_SIZE start_offset,
                       uint32_t options, pcre2_match_data *match_data,
                       pcre2_match_context *mcontext);

       int pcre2_dfa_match(const pcre2_code *code, PCRE2_SPTR subject,
                           PCRE2_SIZE length, PCRE2_SIZE start_offset,
                           uint32_t options, pcre2_match_data *match_data,
                           pcre2_match_context *mcontext, int *workspace,
                           PCRE2_SIZE wscount);

       void pcre2_match_data_free(pcre2_match_data *match_data);


PCRE2 NATIVE API AUXILIARY MATCH FUNCTIONS
       PCRE2_SPTR pcre2_get_mark(pcre2_match_data *match_data);

       PCRE2_SIZE pcre2_get_match_data_size(pcre2_match_data *match_data);

       PCRE2_SIZE pcre2_get_match_data_heapframes_size(
             pcre2_match_data *match_data);

       uint32_t pcre2_get_ovector_count(pcre2_match_data *match_data);

       PCRE2_SIZE *pcre2_get_ovector_pointer(pcre2_match_data *match_data);

       PCRE2_SIZE pcre2_get_startchar(pcre2_match_data *match_data);


PCRE2 NATIVE API GENERAL CONTEXT FUNCTIONS
       pcre2_general_context *pcre2_general_context_create(
             void *(*private_malloc)(size_t size, void *memory_data),
             void (*private_free)(void *ptr, void *memory_data),
             void *memory_data);

       pcre2_general_context *pcre2_general_context_copy(
             pcre2_general_context *gcontext);

       void pcre2_general_context_free(pcre2_general_context *gcontext);


PCRE2 NATIVE API COMPILE CONTEXT FUNCTIONS
       pcre2_compile_context *pcre2_compile_context_create(
             pcre2_general_context *gcontext);

       pcre2_compile_context *pcre2_compile_context_copy(
             pcre2_compile_context *ccontext);

       void pcre2_compile_context_free(pcre2_compile_context *ccontext);

       int pcre2_get_bsr(pcre2_compile_context *ccontext, uint32_t *length);

       int pcre2_get_max_pattern_length(pcre2_compile_context *ccontext,
                                        PCRE2_SIZE *length);

       int pcre2_get_newline(pcre2_compile_context *ccontext,
                             uint32_t *newline);

       int pcre2_get_parens_nest_limit(pcre2_compile_context *ccontext,
                                       uint32_t *limit);

       int pcre2_set_bsr(pcre2_compile_context *ccontext, uint32_t value);

       int pcre2_set_character_tables(pcre2_compile_context *ccontext,
                                      const uint8_t *tables);

       int pcre2_set_compile_extra_options(pcre2_compile_context *ccontext,
                                           uint32_t options);

       int pcre2_set_max_pattern_length(pcre2_compile_context *ccontext,
                                        PCRE2_SIZE length);

       int pcre2_set_max_pattern_compiled_length(
             pcre2_compile_context *ccontext, PCRE2_SIZE length);

       int pcre2_set_max_varlookbehind(pcre2_compile_context *ccontext,
                                       uint32_t limit);

       int pcre2_set_newline(pcre2_compile_context *ccontext,
                             uint32_t newline);

       int pcre2_set_parens_nest_limit(pcre2_compile_context *ccontext,
                                       uint32_t limit);

       int pcre2_set_compile_recursion_guard(pcre2_compile_context *ccontext,
                                             int (*guard)(uint32_t depth,
                                                          void *user_data),
                                             void *user_data);

       int pcre2_set_optimize(pcre2_compile_context *ccontext,
                              uint32_t directive);


PCRE2 NATIVE API MATCH CONTEXT FUNCTIONS
       pcre2_match_context *pcre2_match_context_create(
             pcre2_general_context *gcontext);

       pcre2_match_context *pcre2_match_context_copy(
             pcre2_match_context *mcontext);

       void pcre2_match_context_free(pcre2_match_context *mcontext);

       int pcre2_get_heap_limit(pcre2_match_context *mcontext,
                                uint32_t *limit);

       int pcre2_get_match_limit(pcre2_match_context *mcontext,
                                 uint32_t *limit);

       int pcre2_get_depth_limit(pcre2_match_context *mcontext,
                                 uint32_t *limit);

       int pcre2_get_offset_limit(pcre2_match_context *mcontext,
                                  PCRE2_SIZE *limit);

       int pcre2_set_callout(pcre2_match_context *mcontext,
                             int (*callout)(
                                 pcre2_callout_block *callout_block,
                                 void *callout_data),
                             void *callout_data);

       int pcre2_set_substitute_callout(
             pcre2_match_context *mcontext,
             int (*substitute_callout)(
                 pcre2_substitute_callout_block *callout_block,
                 void *substitute_callout_data),
             void *substitute_callout_data);

       int pcre2_set_substitute_case_callout(
             pcre2_match_context *mcontext,
             PCRE2_SIZE (*substitute_case_callout)(
                 PCRE2_SPTR input, PCRE2_SIZE input_length,
                 PCRE2_UCHAR *output, PCRE2_SIZE output_capacity,
                 int case_operation, void *substitute_case_callout_data),
             void *substitute_case_callout_data);

       int pcre2_set_offset_limit(pcre2_match_context *mcontext,
                                  PCRE2_SIZE limit);

       int pcre2_set_heap_limit(pcre2_match_context *mcontext,
                                uint32_t limit);

       int pcre2_set_match_limit(pcre2_match_context *mcontext,
                                 uint32_t limit);

       int pcre2_set_depth_limit(pcre2_match_context *mcontext,
                                 uint32_t limit);


PCRE2 NATIVE API STRING EXTRACTION FUNCTIONS
       int pcre2_substring_copy_byname(pcre2_match_data *match_data,
                                       PCRE2_SPTR stringname,
                                       PCRE2_UCHAR *buffer,
                                       PCRE2_SIZE *sizeptr);

       int pcre2_substring_copy_bynumber(pcre2_match_data *match_data,
                                         uint32_t stringnumber,
                                         PCRE2_UCHAR *buffer,
                                         PCRE2_SIZE *sizeptr);

       void pcre2_substring_free(PCRE2_UCHAR *string);

       int pcre2_substring_get_byname(pcre2_match_data *match_data,
                                      PCRE2_SPTR stringname,
                                      PCRE2_UCHAR **stringptr,
                                      PCRE2_SIZE *sizeptr);

       int pcre2_substring_get_bynumber(pcre2_match_data *match_data,
                                        uint32_t stringnumber,
                                        PCRE2_UCHAR **stringptr,
                                        PCRE2_SIZE *sizeptr);

       int pcre2_substring_length_byname(pcre2_match_data *match_data,
                                         PCRE2_SPTR stringname,
                                         PCRE2_SIZE *sizeptr);

       int pcre2_substring_length_bynumber(pcre2_match_data *match_data,
                                           uint32_t stringnumber,
                                           PCRE2_SIZE *sizeptr);

       int pcre2_substring_nametable_scan(const pcre2_code *code,
                                          PCRE2_SPTR stringname,
                                          PCRE2_SPTR *firstptr,
                                          PCRE2_SPTR *lastptr);

       int pcre2_substring_number_from_name(const pcre2_code *code,
                                            PCRE2_SPTR stringname);

       void pcre2_substring_list_free(PCRE2_UCHAR **list);

       int pcre2_substring_list_get(pcre2_match_data *match_data,
                                    PCRE2_UCHAR ***listptr,
                                    PCRE2_SIZE **lengthsptr);


PCRE2 NATIVE API STRING SUBSTITUTION FUNCTION
       int pcre2_substitute(const pcre2_code *code, PCRE2_SPTR subject,
                            PCRE2_SIZE length, PCRE2_SIZE start_offset,
                            uint32_t options, pcre2_match_data *match_data,
                            pcre2_match_context *mcontext,
                            PCRE2_SPTR replacement, PCRE2_SIZE rlength,
                            PCRE2_UCHAR *buffer, PCRE2_SIZE *blength);


PCRE2 NATIVE API JIT FUNCTIONS
       int pcre2_jit_compile(pcre2_code *code, uint32_t options);

       int pcre2_jit_match(const pcre2_code *code, PCRE2_SPTR subject,
                           PCRE2_SIZE length, PCRE2_SIZE start_offset,
                           uint32_t options, pcre2_match_data *match_data,
                           pcre2_match_context *mcontext);

       void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);

       pcre2_jit_stack *pcre2_jit_stack_create(
             size_t startsize, size_t maxsize,
             pcre2_general_context *gcontext);

       void pcre2_jit_stack_assign(pcre2_match_context *mcontext,
                                   pcre2_jit_callback callback,
                                   void *callback_data);

       void pcre2_jit_stack_free(pcre2_jit_stack *jit_stack);


PCRE2 NATIVE API SERIALIZATION FUNCTIONS
       int32_t pcre2_serialize_decode(pcre2_code **codes,
                                      int32_t number_of_codes,
                                      const uint8_t *bytes,
                                      pcre2_general_context *gcontext);

       int32_t pcre2_serialize_encode(const pcre2_code **codes,
                                      int32_t number_of_codes,
                                      uint8_t **serialized_bytes,
                                      PCRE2_SIZE *serialized_size,
                                      pcre2_general_context *gcontext);

       void pcre2_serialize_free(uint8_t *bytes);

       int32_t pcre2_serialize_get_number_of_codes(const uint8_t *bytes);


PCRE2 NATIVE API AUXILIARY FUNCTIONS
       pcre2_code *pcre2_code_copy(const pcre2_code *code);

       pcre2_code *pcre2_code_copy_with_tables(const pcre2_code *code);

       int pcre2_get_error_message(int errorcode, PCRE2_UCHAR *buffer,
                                   PCRE2_SIZE bufflen);

       const uint8_t *pcre2_maketables(pcre2_general_context *gcontext);

       void pcre2_maketables_free(pcre2_general_context *gcontext,
                                  const uint8_t *tables);

       int pcre2_pattern_info(const pcre2_code *code, uint32_t what,
                              void *where);

       int pcre2_callout_enumerate(
             const pcre2_code *code,
             int (*callback)(pcre2_callout_enumerate_block *callout_block,
                             void *callout_data),
             void *callout_data);

       int pcre2_config(uint32_t what, void *where);


PCRE2 NATIVE API OBSOLETE FUNCTIONS
       int pcre2_set_recursion_limit(pcre2_match_context *mcontext,
                                     uint32_t limit);

       int pcre2_set_recursion_memory_management(
             pcre2_match_context *mcontext,
             void *(*mymalloc)(size_t size, void *mydata),
             void (*myfree)(void *ptr, void *mydata),
             void *mydata);

       These functions became obsolete at release 10.30 and are retained only
       for backward compatibility. They should not be used in new code. The
       first is replaced by pcre2_set_depth_limit(); the second is no longer
       needed and has no effect (it always returns zero).


PCRE2 EXPERIMENTAL PATTERN CONVERSION FUNCTIONS
       pcre2_convert_context *pcre2_convert_context_create(
             pcre2_general_context *gcontext);

       pcre2_convert_context *pcre2_convert_context_copy(
             pcre2_convert_context *ccontext);

       void pcre2_convert_context_free(pcre2_convert_context *ccontext);

       int pcre2_set_glob_escape(pcre2_convert_context *ccontext,
                                 uint32_t escape_char);

       int pcre2_set_glob_separator(pcre2_convert_context *ccontext,
                                    uint32_t separator_char);

       int pcre2_pattern_convert(PCRE2_SPTR pattern, PCRE2_SIZE plength,
                                 uint32_t options, PCRE2_UCHAR **buffptr,
                                 PCRE2_SIZE *bufflenptr,
                                 pcre2_convert_context *ccontext);

       void pcre2_converted_pattern_free(PCRE2_UCHAR *converted);

       These functions provide a way of converting non-PCRE2 patterns into
       patterns that can be processed by pcre2_compile(). This facility is
       experimental and may be changed in future releases. At present, "globs"
       and POSIX basic and extended patterns can be converted. Details are
       given in the pcre2convert documentation.


PCRE2 8-BIT, 16-BIT, AND 32-BIT LIBRARIES
       There are three PCRE2 libraries, supporting 8-bit, 16-bit, and 32-bit
       code units, respectively. However, there is just one header file,
       pcre2.h. This contains the function prototypes and other definitions
       for all three libraries. One, two, or all three can be installed
       simultaneously. On Unix-like systems the libraries are called
       libpcre2-8, libpcre2-16, and libpcre2-32, and they can also co-exist
       with the original PCRE libraries. Every PCRE2 function comes in three
       different forms, one for each library, for example:

         pcre2_compile_8()
         pcre2_compile_16()
         pcre2_compile_32()

       There are also three different sets of data types:

         PCRE2_UCHAR8, PCRE2_UCHAR16, PCRE2_UCHAR32
         PCRE2_SPTR8,  PCRE2_SPTR16,  PCRE2_SPTR32

       The UCHAR types define unsigned code units of the appropriate widths.
       For example, PCRE2_UCHAR16 is usually defined as uint16_t. The SPTR
       types are pointers to constants of the equivalent UCHAR types, that is,
       they are pointers to vectors of unsigned code units.

       Character strings are passed to a PCRE2 library as sequences of
       unsigned integers in code units of the appropriate width. The length of
       a string may be given as a number of code units, or the string may be
       specified as zero-terminated.

       Many applications use only one code unit width. For their convenience,
       macros are defined whose names are the generic forms such as
       pcre2_compile() and PCRE2_SPTR. These macros use the value of the macro
       PCRE2_CODE_UNIT_WIDTH to generate the appropriate width-specific
       function and macro names. PCRE2_CODE_UNIT_WIDTH is not defined by
       default. An application must define it to be 8, 16, or 32 before
       including pcre2.h in order to make use of the generic names.

       Applications that use more than one code unit width can be linked with
       more than one PCRE2 library, but must define PCRE2_CODE_UNIT_WIDTH to
       be 0 before including pcre2.h, and then use the real function names.
       Any code that is to be included in an environment where the value of
       PCRE2_CODE_UNIT_WIDTH is unknown should also use the real function
       names. (Unfortunately, it is not possible in C code to save and restore
       the value of a macro.)

       If PCRE2_CODE_UNIT_WIDTH is not defined before including pcre2.h, a
       compiler error occurs.

       When using multiple libraries in an application, you must take care
       when processing any particular pattern to use only functions from a
       single library. For example, if you want to run a match using a pattern
       that was compiled with pcre2_compile_16(), you must do so with
       pcre2_match_16(), not pcre2_match_8() or pcre2_match_32().

       In the function summaries above, and in the rest of this document and
       other PCRE2 documents, functions and data types are described using
       their generic names, without the _8, _16, or _32 suffix.


PCRE2 API OVERVIEW
       PCRE2 has its own native API, which is described in this document.
       There are also some wrapper functions for the 8-bit library that
       correspond to the POSIX regular expression API, but they do not give
       access to all the functionality of PCRE2 and they are not thread-safe.
       They are described in the pcre2posix documentation. Both these APIs
       define a set of C function calls.

       The native API C data types, function prototypes, option values, and
       error codes are defined in the header file pcre2.h, which also contains
       definitions of PCRE2_MAJOR and PCRE2_MINOR, the major and minor release
       numbers for the library. Applications can use these to include support
       for different releases of PCRE2.

       In a Windows environment, if you want to statically link an application
       program against a non-dll PCRE2 library, you must define PCRE2_STATIC
       before including pcre2.h.

       The functions pcre2_compile() and pcre2_match() are used for compiling
       and matching regular expressions in a Perl-compatible manner. A sample
       program that demonstrates the simplest way of using them is provided in
       the file called pcre2demo.c in the PCRE2 source distribution. A listing
       of this program is given in the pcre2demo documentation, and the
       pcre2sample documentation describes how to compile and run it.

       The compiling and matching functions recognize various options that are
       passed as bits in an options argument. There are also some more
       complicated parameters such as custom memory management functions and
       resource limits that are passed in "contexts" (which are just memory
       blocks, described below). Simple applications do not need to make use
       of contexts.

       Just-in-time (JIT) compiler support is an optional feature of PCRE2
       that can be built in appropriate hardware environments. It greatly
       speeds up the matching performance of many patterns. Programs can
       request that it be used if available by calling pcre2_jit_compile()
       after a pattern has been successfully compiled by pcre2_compile(). This
       does nothing if JIT support is not available.

       More complicated programs might need to make use of the specialist
       functions pcre2_jit_stack_create(), pcre2_jit_stack_free(), and
       pcre2_jit_stack_assign() in order to control the JIT code's memory
       usage.

       JIT matching is automatically used by pcre2_match() if it is available,
       unless the PCRE2_NO_JIT option is set. There is also a direct interface
       for JIT matching, which gives improved performance at the expense of
       less sanity checking. The JIT-specific functions are discussed in the
       pcre2jit documentation.

       A second matching function, pcre2_dfa_match(), which is not
       Perl-compatible, is also provided. This uses a different algorithm for
       the matching. The alternative algorithm finds all possible matches (at
       a given point in the subject), and scans the subject just once (unless
       there are lookaround assertions). However, this algorithm does not
       return captured substrings. A description of the two matching
       algorithms and their advantages and disadvantages is given in the
       pcre2matching documentation. There is no JIT support for
       pcre2_dfa_match().

       In addition to the main compiling and matching functions, there are
       convenience functions for extracting captured substrings from a subject
       string that has been matched by pcre2_match(). They are:

         pcre2_substring_copy_byname()
         pcre2_substring_copy_bynumber()
         pcre2_substring_get_byname()
         pcre2_substring_get_bynumber()
         pcre2_substring_list_get()
         pcre2_substring_length_byname()
         pcre2_substring_length_bynumber()
         pcre2_substring_nametable_scan()
         pcre2_substring_number_from_name()

       pcre2_substring_free() and pcre2_substring_list_free() are also
       provided, to free memory used for extracted strings. If either of these
       functions is called with a NULL argument, the function returns
       immediately without doing anything.

       The function pcre2_substitute() can be called to match a pattern and
       return a copy of the subject string with substitutions for parts that
       were matched.

       Functions whose names begin with pcre2_serialize_ are used for saving
       compiled patterns on disc or elsewhere, and reloading them later.

       Finally, there are functions for finding out information about a
       compiled pattern (pcre2_pattern_info()) and about the configuration
       with which PCRE2 was built (pcre2_config()) and that it is using.

       Functions with names ending with _free() are used for freeing memory
       blocks of various sorts. In all cases, if one of these functions is
       called with a NULL argument, it does nothing.


STRING LENGTHS AND OFFSETS
       The PCRE2 API uses string lengths and offsets into strings of code
       units in several places. These values are always of type PCRE2_SIZE,
       which is an unsigned integer type, currently always defined as size_t.
       The largest value that can be stored in such a type (that is
       ~(PCRE2_SIZE)0) is reserved as a special indicator for zero-terminated
       strings and unset offsets. Therefore, the longest string that can be
       handled is one less than this maximum. Note that string lengths are
       always given in code units. Only in the 8-bit library is such a length
       the same as the number of bytes in the string.


NEWLINES
       PCRE2 supports five different conventions for indicating line breaks in
       strings: a single CR (carriage return) character, a single LF
       (linefeed) character, the two-character sequence CRLF, any of the three
       preceding, or any Unicode newline sequence. The Unicode newline
       sequences are the three just mentioned, plus the single characters VT
       (vertical tab, U+000B), FF (form feed, U+000C), NEL (next line,
       U+0085), LS (line separator, U+2028), and PS (paragraph separator,
       U+2029).

       Each of the first three conventions is used by at least one operating
       system as its standard newline sequence. When PCRE2 is built, a default
       can be specified. If it is not, the default is set to LF, which is the
       Unix standard. However, the newline convention can be changed by an
       application when calling pcre2_compile(), or it can be specified by
       special text at the start of the pattern itself; this overrides any
       other settings. See the pcre2pattern page for details of the special
       character sequences.

       In the PCRE2 documentation the word "newline" is used to mean "the
       character or pair of characters that indicate a line break". The choice
       of newline convention affects the handling of the dot, circumflex, and
       dollar metacharacters, the handling of #-comments in /x mode, and, when
       CRLF is a recognized line ending sequence, the match position
       advancement for a non-anchored pattern. There is more detail about this
       in the section on pcre2_match() options below.

       The choice of newline convention does not affect the interpretation of
       the \n or \r escape sequences, nor does it affect what \R matches; this
       has its own separate convention.


MULTITHREADING
       In a multithreaded application it is important to keep thread-specific
       data separate from data that can be shared between threads. The PCRE2
       library code itself is thread-safe: it contains no static or global
       variables. The API is designed to be fairly simple for non-threaded
       applications while at the same time ensuring that multithreaded
       applications can use it.

       There are several different blocks of data that are used to pass
       information between the application and the PCRE2 libraries.

   The compiled pattern
       A pointer to the compiled form of a pattern is returned to the user
       when pcre2_compile() is successful. The data in the compiled pattern is
       fixed, and does not change when the pattern is matched. Therefore, it
       is thread-safe, that is, the same compiled pattern can be used by more
       than one thread simultaneously. For example, an application can compile
       all its patterns at the start, before forking off multiple threads that
       use them. However, if the just-in-time (JIT) optimization feature is
       being used, it needs separate memory stack areas for each thread. See
       the pcre2jit documentation for more details.

       In a more complicated situation, where patterns are compiled only when
       they are first needed, but are still shared between threads, pointers
       to compiled patterns must be protected from simultaneous writing by
       multiple threads. This is somewhat tricky to do correctly. If you know
       that writing to a pointer is atomic in your environment, you can use
       logic like this:

       Get a read-only (shared) lock (mutex) for pointer
       if (pointer == NULL)
         {
         Get a write (unique) lock for pointer
         if (pointer == NULL) pointer = pcre2_compile(...
         }
       Release the lock
       Use pointer in pcre2_match()

       Of course, testing for compilation errors should also be included in
       the code.

       The reason for checking the pointer a second time is as follows:
       Several threads may have acquired the shared lock and tested the
       pointer for being NULL, but only one of them will be given the write
       lock, with the rest kept waiting. The winning thread will compile the
       pattern and store the result. After this thread releases the write
       lock, another thread will get it, and if it does not retest pointer for
       being NULL, will recompile the pattern and overwrite the pointer,
       creating a memory leak and possibly causing other issues.

       In an environment where writing to a pointer may not be atomic, the
       above logic is not sufficient. The thread that is doing the compiling
       may be descheduled after writing only part of the pointer, which could
       cause other threads to use an invalid value. Instead of checking the
       pointer itself, a separate "pointer is valid" flag (that can be updated
       atomically) must be used:

       Get a read-only (shared) lock (mutex) for pointer
       if (!pointer_is_valid)
         {
         Get a write (unique) lock for pointer
         if (!pointer_is_valid)
           {
           pointer = pcre2_compile(...
           pointer_is_valid = TRUE
           }
         }
       Release the lock
       Use pointer in pcre2_match()

       If JIT is being used, but the JIT compilation is not being done
       immediately (perhaps waiting to see if the pattern is used often
       enough), similar logic is required. JIT compilation updates a value
       within the compiled code block, so a thread must gain unique write
       access to the pointer before calling pcre2_jit_compile().
       Alternatively, pcre2_code_copy() or pcre2_code_copy_with_tables() can
       be used to obtain a private copy of the compiled code before calling
       the JIT compiler.

   Context blocks
       The next main section below introduces the idea of "contexts" in which
       PCRE2 functions are called. A context is nothing more than a collection
       of parameters that control the way PCRE2 operates. Grouping a number of
       parameters together in a context is a convenient way of passing them to
       a PCRE2 function without using lots of arguments. The parameters that
       are stored in contexts are in some sense "advanced features" of the
       API. Many straightforward applications will not need to use contexts.

       In a multithreaded application, if the parameters in a context are
       values that are never changed, the same context can be used by all the
       threads. However, if any thread needs to change any value in a context,
       it must make its own thread-specific copy.

   Match blocks
       The matching functions need a block of memory for storing the results
       of a match. This includes details of what was matched, as well as
       additional information such as the name of a (*MARK) setting. Each
       thread must provide its own copy of this memory.


PCRE2 CONTEXTS
       Some PCRE2 functions have a lot of parameters, many of which are used
       only by specialist applications, for example, those that use custom
       memory management or non-standard character tables. To keep function
       argument lists at a reasonable size, and at the same time to keep the
       API extensible, "uncommon" parameters are passed to certain functions
       in a context instead of directly. A context is just a block of memory
       that holds the parameter values. Applications that do not need to
       adjust any of the context parameters can pass NULL when a context
       pointer is required.

       There are three different types of context: a general context that is
       relevant for several PCRE2 operations, a compile-time context, and a
       match-time context.

   The general context
       At present, this context just contains pointers to (and data for)
       external memory management functions that are called from several
       places in the PCRE2 library. The context is named `general' rather than
       specifically `memory' because in future other fields may be added. If
       you do not want to supply your own custom memory management functions,
       you do not need to bother with a general context. A general context is
       created by:

       pcre2_general_context *pcre2_general_context_create(
             void *(*private_malloc)(size_t size, void *memory_data),
             void (*private_free)(void *ptr, void *memory_data),
             void *memory_data);

       The two function pointers specify custom memory management functions,
       whose prototypes are:

         void *private_malloc(PCRE2_SIZE, void *);
         void  private_free(void *, void *);

       Whenever code in PCRE2 calls these functions, the final argument is the
       value of memory_data. Either of the first two arguments of the creation
       function may be NULL, in which case the system memory management
       functions malloc() and free() are used. (This is not currently useful,
       as there are no other fields in a general context, but in future there
       might be.) The private_malloc() function is used (if supplied) to
       obtain memory for storing the context, and all three values are saved
       as part of the context.

       Whenever PCRE2 creates a data block of any kind, the block contains a
       pointer to the free() function that matches the malloc() function that
       was used. When the time comes to free the block, this function is
       called.

       A general context can be copied by calling:

       pcre2_general_context *pcre2_general_context_copy(
             pcre2_general_context *gcontext);

       The memory used for a general context should be freed by calling:

       void pcre2_general_context_free(pcre2_general_context *gcontext);

       If this function is passed a NULL argument, it returns immediately
       without doing anything.

   The compile context
       A compile context is required if you want to provide an external
       function for stack checking during compilation or to change the default
       values of any of the following compile-time parameters:

       o   What \R matches (Unicode newlines or CR, LF, CRLF only)

       o   PCRE2's character tables

       o   The newline character sequence

       o   The compile time nested parentheses limit

       o   The maximum length of the pattern string

       o   The extra options bits (none set by default)

       o   Which performance optimizations the compiler should apply

       A compile context is also required if you are using custom memory
       management. If none of these apply, just pass NULL as the context
       argument of pcre2_compile().

       A compile context is created, copied, and freed by the following
       functions:

       pcre2_compile_context *pcre2_compile_context_create(
             pcre2_general_context *gcontext);

       pcre2_compile_context *pcre2_compile_context_copy(
             pcre2_compile_context *ccontext);

       void pcre2_compile_context_free(pcre2_compile_context *ccontext);

       A compile context is created with default values for its parameters.
       These can be changed by calling the following functions, which return 0
       on success, or PCRE2_ERROR_BADDATA if invalid data is detected.

       The following functions get, respectively, the convention for
       processing \R, the maximum pattern length, the newline convention, and
       the maximum depth of nested parentheses. They return 0 on success or
       PCRE2_ERROR_NULL if either argument is NULL.

       int pcre2_get_bsr(pcre2_compile_context *ccontext, uint32_t *length);

       int pcre2_get_max_pattern_length(pcre2_compile_context *ccontext,
                                        PCRE2_SIZE *length);

       int pcre2_get_newline(pcre2_compile_context *ccontext,
                             uint32_t *newline);

       int pcre2_get_parens_nest_limit(pcre2_compile_context *ccontext,
                                       uint32_t *limit);

       The value returned by pcre2_get_max_pattern_length() is the largest
       value a PCRE2_SIZE variable can hold when the limit is unlimited.
       Passing this value to pcre2_set_max_pattern_length() makes the limit
       unlimited.

       int pcre2_set_bsr(pcre2_compile_context *ccontext, uint32_t value);

       The value must be PCRE2_BSR_ANYCRLF, to specify that \R matches only
       CR, LF, or CRLF, or PCRE2_BSR_UNICODE, to specify that \R matches any
       Unicode line ending sequence. The value is used by the JIT compiler and
       by the two interpreted matching functions, pcre2_match() and
       pcre2_dfa_match().

       int pcre2_set_character_tables(pcre2_compile_context *ccontext,
                                      const uint8_t *tables);

       The value must be the result of a call to pcre2_maketables(), whose
       only argument is a general context. This function builds a set of
       character tables in the current locale.

       int pcre2_set_compile_extra_options(pcre2_compile_context *ccontext,
                                           uint32_t options);

       As PCRE2 has developed, almost all the 32 option bits that are
       available in the options argument of pcre2_compile() have been used up.
       To avoid running out, the compile context contains a set of extra
       option bits which are used for some newer, assumed rarer, options. This
       function sets those bits. It always sets all the bits (either on or
       off). It does not modify any existing setting. The available options
       are defined in the section entitled "Extra compile options" below.

       int pcre2_set_max_pattern_length(pcre2_compile_context *ccontext,
                                        PCRE2_SIZE length);

       This sets a maximum length, in code units, for any pattern string that
       is compiled with this context. If the pattern is longer, an error is
       generated. This facility is provided so that applications that accept
       patterns from external sources can limit their size. The default is the
       largest number that a PCRE2_SIZE variable can hold, which is
       effectively unlimited.

       int pcre2_set_max_pattern_compiled_length(
             pcre2_compile_context *ccontext, PCRE2_SIZE length);

       This sets a maximum size, in bytes, for the memory needed to hold the
       compiled version of a pattern that is compiled with this context. If
       the pattern needs more memory, an error is generated. This facility is
       provided so that applications that accept patterns from external
       sources can limit the amount of memory they use. The default is the
       largest number that a PCRE2_SIZE variable can hold, which is
       effectively unlimited.

       int pcre2_set_max_varlookbehind(pcre2_compile_context *ccontext,
                                       uint32_t limit);

       This sets a maximum length for the number of characters matched by a
       variable-length lookbehind assertion. The default is set when PCRE2 is
       built, with the ultimate default being 255, the same as Perl.
       Lookbehind assertions without a bounding length are not supported.

       int pcre2_set_newline(pcre2_compile_context *ccontext,
                             uint32_t newline);

       This specifies which characters or character sequences are to be
       recognized as newlines. The value must be one of PCRE2_NEWLINE_CR
       (carriage return only), PCRE2_NEWLINE_LF (linefeed only),
       PCRE2_NEWLINE_CRLF (the two-character sequence CR followed by LF),
       PCRE2_NEWLINE_ANYCRLF (any of the above), PCRE2_NEWLINE_ANY (any
       Unicode newline sequence), or PCRE2_NEWLINE_NUL (the NUL character,
       that is a binary zero).

       A pattern can override the value set in the compile context by starting
       with a sequence such as (*CRLF). See the pcre2pattern page for details.

       When a pattern is compiled with the PCRE2_EXTENDED or
       PCRE2_EXTENDED_MORE option, the newline convention affects the
       recognition of the end of internal comments starting with #. The value
       is saved with the compiled pattern for subsequent use by the JIT
       compiler and by the two interpreted matching functions, pcre2_match()
       and pcre2_dfa_match().

       int pcre2_set_parens_nest_limit(pcre2_compile_context *ccontext,
                                       uint32_t limit);

       This parameter adjusts the limit, set when PCRE2 is built (default
       250), on the depth of parenthesis nesting in a pattern. This limit
       stops rogue patterns using up too much system stack when being
       compiled. The limit applies to parentheses of all kinds, not just
       capturing parentheses.

       int pcre2_set_compile_recursion_guard(pcre2_compile_context *ccontext,
                                             int (*guard)(uint32_t depth,
                                                          void *user_data),
                                             void *user_data);

       There is at least one application that runs PCRE2 in threads with very
       limited system stack, where running out of stack is to be avoided at
       all costs. The parenthesis limit above cannot take account of how much
       stack is actually available during compilation. For a finer control,
       you can supply a function that is called whenever pcre2_compile()
       starts to compile a parenthesized part of a pattern. This function can
       check the actual stack size (or anything else that it wants to, of
       course).

       The first argument to the callout function gives the current depth of
       nesting, and the second is user data that is set up by the last
       argument of pcre2_set_compile_recursion_guard(). The callout function
       should return zero if all is well, or non-zero to force an error.

       int pcre2_set_optimize(pcre2_compile_context *ccontext,
                              uint32_t directive);

       PCRE2 can apply various performance optimizations during compilation,
       in order to make matching faster. For example, the compiler might
       convert some regex constructs into an equivalent construct which
       pcre2_match() can execute faster. By default, all available
       optimizations are enabled. However, in rare cases, one might wish to
       disable specific optimizations. For example, if it is known that some
       optimizations cannot benefit a certain regex, it might be desirable to
       disable them, in order to speed up compilation.

       The permitted values of directive are as follows:

         PCRE2_OPTIMIZATION_FULL

       Enable all optional performance optimizations. This is the default
       value.

         PCRE2_OPTIMIZATION_NONE

       Disable all optional performance optimizations.

         PCRE2_AUTO_POSSESS
         PCRE2_AUTO_POSSESS_OFF

       Enable/disable "auto-possessification" of variable quantifiers such as
       * and +. This optimization, for example, turns a+b into a++b in order
       to avoid backtracks into a++ that can never be successful. However, if
       callouts are in use, auto-possessification means that some callouts are
       never taken. You can disable this optimization if you want the matching
       functions to do a full, unoptimized search and run all the callouts.

         PCRE2_DOTSTAR_ANCHOR
         PCRE2_DOTSTAR_ANCHOR_OFF

       Enable/disable an optimization that is applied when . is the first
       significant item in a top-level branch of a pattern, and all the other
       branches also start with . or with \A or \G or ^. Such a pattern is
       automatically anchored if PCRE2_DOTALL is set for all the .* items and
       PCRE2_MULTILINE is not set for any ^ items. Otherwise, the fact that
       any match must start either at the start of the subject or following a
       newline is remembered. Like other optimizations, this can cause
       callouts to be skipped.

       Dotstar anchor optimization is automatically disabled for .* if it is
       inside an atomic group or a capture group that is the subject of a
       backreference, or if the pattern contains (*PRUNE) or (*SKIP).

         PCRE2_START_OPTIMIZE
         PCRE2_START_OPTIMIZE_OFF

       Enable/disable optimizations which cause matching functions to scan the
       subject string for specific code unit values before attempting a match.
       For example, if it is known that an unanchored match must start with a
       specific value, the matching code searches the subject for that value,
       and fails immediately if it cannot find it, without actually running
       the main matching function. This means that a special item such as
       (*COMMIT) at the start of a pattern is not considered until after a
       suitable starting point for the match has been found. Also, when
       callouts or (*MARK) items are in use, these "start-up" optimizations
       can cause them to be skipped if the pattern is never actually used. The
       start-up optimizations are in effect a pre-scan of the subject that
       takes place before the pattern is run.

       Disabling start-up optimizations ensures that in cases where the result
       is "no match", the callouts do occur, and that items such as (*COMMIT)
       and (*MARK) are considered at every possible starting position in the
       subject string.

       Disabling start-up optimizations may change the outcome of a matching
       operation. Consider the pattern

         (*COMMIT)ABC

       When this is compiled, PCRE2 records the fact that a match must start
       with the character "A". Suppose the subject string is "DEFABC". The
       start-up optimization scans along the subject, finds "A" and runs the
       first match attempt from there. The (*COMMIT) item means that the
       pattern must match the current starting position, which in this case,
       it does. However, if the same match is run without start-up
       optimizations, the initial scan along the subject string does not
       happen. The first match attempt is run starting from "D" and when this
       fails, (*COMMIT) prevents any further matches being tried, so the
       overall result is "no match".

       Another start-up optimization makes use of a minimum length for a
       matching subject, which is recorded when possible. Consider the pattern

         (*MARK:1)B(*MARK:2)(X|Y)

       The minimum length for a match is two characters. If the subject is
       "XXBB", the "starting character" optimization skips "XX", then tries to
       match "BB", which is long enough. In the process, (*MARK:2) is
       encountered and remembered. When the match attempt fails, the next "B"
       is found, but there is only one character left, so there are no more
       attempts, and "no match" is returned with the "last mark seen" set to
       "2". Without start-up optimizations, however, matches are tried at
       every possible starting position, including at the end of the subject,
       where (*MARK:1) is encountered, but there is no "B", so the "last mark
       seen" that is returned is "1". In this case, the optimizations do not
       affect the overall match result, which is still "no match", but they do
       affect the auxiliary information that is returned.

   The match context
       A match context is required if you want to:

         Set up a callout function
         Set an offset limit for matching an unanchored pattern
         Change the limit on the amount of heap used when matching
         Change the backtracking match limit
         Change the backtracking depth limit
         Set custom memory management specifically for the match

       If none of these apply, just pass NULL as the context argument of
       pcre2_match(), pcre2_dfa_match(), or pcre2_jit_match().

       A match context is created, copied, and freed by the following
       functions:

       pcre2_match_context *pcre2_match_context_create(
             pcre2_general_context *gcontext);

       pcre2_match_context *pcre2_match_context_copy(
             pcre2_match_context *mcontext);

       void pcre2_match_context_free(pcre2_match_context *mcontext);

       A match context is created with default values for its parameters.
       These can be changed by calling the following functions, which return 0
       on success, or PCRE2_ERROR_BADDATA if invalid data is detected.

       The following functions get, respectively, the backtracking heap limit,
       match limit, depth limit, and offset limit. They return 0 on success or
       PCRE2_ERROR_NULL if either argument is NULL.

       int pcre2_get_heap_limit(pcre2_match_context *mcontext,
                                uint32_t *limit);

       int pcre2_get_match_limit(pcre2_match_context *mcontext,
                                 uint32_t *limit);

       int pcre2_get_depth_limit(pcre2_match_context *mcontext,
                                 uint32_t *limit);

       int pcre2_get_offset_limit(pcre2_match_context *mcontext,
                                  PCRE2_SIZE *limit);

       The value returned by pcre2_get_offset_limit() is the largest value a
       PCRE2_SIZE variable can hold when there is no offset limit. Passing
       this value to pcre2_set_offset_limit() removes the offset limit.

       int pcre2_set_callout(pcre2_match_context *mcontext,
                             int (*callout)(
                                 pcre2_callout_block *callout_block,
                                 void *callout_data),
                             void *callout_data);

       This sets up a callout function for PCRE2 to call at specified points
       during a matching operation. Details are given in the pcre2callout
       documentation.

       int pcre2_set_substitute_callout(
             pcre2_match_context *mcontext,
             int (*substitute_callout)(
                 pcre2_substitute_callout_block *callout_block,
                 void *substitute_callout_data),
             void *substitute_callout_data);

       This sets up a callout function for PCRE2 to call after each
       substitution made by pcre2_substitute(). Details are given in the
       section entitled "Creating a new string with substitutions" below.

       int pcre2_set_substitute_case_callout(
             pcre2_match_context *mcontext,
             PCRE2_SIZE (*substitute_case_callout)(
                 PCRE2_SPTR input, PCRE2_SIZE input_length,
                 PCRE2_UCHAR *output, PCRE2_SIZE output_capacity,
                 int case_operation, void *substitute_case_callout_data),
             void *substitute_case_callout_data);

       This sets up a callout function for PCRE2 to call when performing case
       transformations inside pcre2_substitute(). Details are given in the
       section entitled "Creating a new string with substitutions" below.

       int pcre2_set_offset_limit(pcre2_match_context *mcontext,
                                  PCRE2_SIZE limit);

       The offset_limit parameter limits how far an unanchored search can
       advance in the subject string. The default value is PCRE2_UNSET. The
       pcre2_match() and pcre2_dfa_match() functions return
       PCRE2_ERROR_NOMATCH if a match with a starting point before or at the
       given offset is not found. The pcre2_substitute() function makes no
       more substitutions.

       For example, if the pattern /abc/ is matched against "123abc" with an
       offset limit less than 3, the result is PCRE2_ERROR_NOMATCH. A match
       can never be found if the start_offset argument of pcre2_match(),
       pcre2_dfa_match(), or pcre2_substitute() is greater than the offset
       limit set in the match context.

       When using this facility, you must set the PCRE2_USE_OFFSET_LIMIT
       option when calling pcre2_compile() so that when JIT is in use,
       different code can be compiled. If a match is started with a
       non-default match limit when PCRE2_USE_OFFSET_LIMIT is not set, an
       error is generated.

       The offset limit facility can be used to track progress when searching
       large subject strings or to limit the extent of global substitutions.
       See also the PCRE2_FIRSTLINE option, which requires a match to start
       before or at the first newline that follows the start of matching in
       the subject. If this is set with an offset limit, a match must occur in
       the first line and also within the offset limit. In other words,
       whichever limit comes first is used.

       int pcre2_set_heap_limit(pcre2_match_context *mcontext,
                                uint32_t limit);

       The heap_limit parameter specifies, in units of kibibytes (1024 bytes),
       the maximum amount of heap memory that pcre2_match() may use to hold
       backtracking information when running an interpretive match. This limit
       also applies to pcre2_dfa_match(), which may use the heap when
       processing patterns with a lot of nested pattern recursion or
       lookarounds or atomic groups. This limit does not apply to matching
       with the JIT optimization, which has its own memory control
       arrangements (see the pcre2jit documentation for more details). If the
       limit is reached, the negative error code PCRE2_ERROR_HEAPLIMIT is
       returned. The default limit can be set when PCRE2 is built; if it is
       not, the default is set very large and is essentially unlimited.

       A value for the heap limit may also be supplied by an item at the start
       of a pattern of the form

       (*LIMIT_HEAP=ddd)

       where ddd is a decimal number. However, such a setting is ignored
       unless ddd is less than the limit set by the caller of pcre2_match()
       or, if no such limit is set, less than the default.

       The pcre2_match() function always needs some heap memory, so setting a
       value of zero guarantees a "heap limit exceeded" error. Details of how
       pcre2_match() uses the heap are given in the pcre2perform
       documentation.

       For pcre2_dfa_match(), a vector on the system stack is used when
       processing pattern recursions, lookarounds, or atomic groups, and only
       if this is not big enough is heap memory used. In this case, setting a
       value of zero disables the use of the heap.

       int pcre2_set_match_limit(pcre2_match_context *mcontext,
                                 uint32_t limit);

       The match_limit parameter provides a means of preventing PCRE2 from
       using up too many computing resources when processing patterns that are
       not going to match, but which have a very large number of possibilities
       in their search trees. The classic example is a pattern that uses
       nested unlimited repeats.

       There is an internal counter in pcre2_match() that is incremented each
       time round its main matching loop. If this value reaches the match
       limit, pcre2_match() returns the negative value PCRE2_ERROR_MATCHLIMIT.
       This has the effect of limiting the amount of backtracking that can
       take place. For patterns that are not anchored, the count restarts from
       zero for each position in the subject string. This limit also applies
       to pcre2_dfa_match(), though the counting is done in a different way.

       When pcre2_match() is called with a pattern that was successfully
       processed by pcre2_jit_compile(), the way in which matching is executed
       is entirely different. However, there is still the possibility of
       runaway matching that goes on for a very long time, and so the
       match_limit value is also used in this case (but in a different way) to
       limit how long the matching can continue.

       The default value for the limit can be set when PCRE2 is built; the
       default is 10 million, which handles all but the most extreme cases. A
       value for the match limit may also be supplied by an item at the start
       of a pattern of the form

       (*LIMIT_MATCH=ddd)

       where ddd is a decimal number. However, such a setting is ignored
       unless ddd is less than the limit set by the caller of pcre2_match() or
       pcre2_dfa_match() or, if no such limit is set, less than the default.

       int pcre2_set_depth_limit(pcre2_match_context *mcontext,
                                 uint32_t limit);

       This parameter limits the depth of nested backtracking in
       pcre2_match(). Each time a nested backtracking point is passed, a new
       memory frame is used to remember the state of matching at that point.
       Thus, this parameter indirectly limits the amount of memory that is
       used in a match. However, because the size of each memory frame depends
       on the number of capturing parentheses, the actual memory limit varies
       from pattern to pattern. This limit was more useful in versions before
       10.30, where function recursion was used for backtracking.

       The depth limit is not relevant, and is ignored, when matching is done
       using JIT compiled code. However, it is supported by pcre2_dfa_match(),
       which uses it to limit the depth of nested internal recursive function
       calls that implement atomic groups, lookaround assertions, and pattern
       recursions. This limits, indirectly, the amount of system stack that is
       used. It was more useful in versions before 10.32, when stack memory
       was used for local workspace vectors for recursive function calls. From
       version 10.32, only local variables are allocated on the stack and as
       each call uses only a few hundred bytes, even a small stack can support
       quite a lot of recursion.

       If the depth of internal recursive function calls is great enough,
       local workspace vectors are allocated on the heap from version 10.32
       onwards, so the depth limit also indirectly limits the amount of heap
       memory that is used. A recursive pattern such as /(.(?2))((?1)|)/, when
       matched to a very long string using pcre2_dfa_match(), can use a great
       deal of memory. However, it is probably better to limit heap usage
       directly by calling pcre2_set_heap_limit().

       The default value for the depth limit can be set when PCRE2 is built;
       if it is not, the default is set to the same value as the default for
       the match limit. If the limit is exceeded, pcre2_match() or
       pcre2_dfa_match() returns PCRE2_ERROR_DEPTHLIMIT. A value for the depth
       limit may also be supplied by an item at the start of a pattern of the
       form

       (*LIMIT_DEPTH=ddd)

       where ddd is a decimal number. However, such a setting is ignored
       unless ddd is less than the limit set by the caller of pcre2_match() or
       pcre2_dfa_match() or, if no such limit is set, less than the default.


CHECKING BUILD-TIME OPTIONS
       int pcre2_config(uint32_t what, void *where);

       The function pcre2_config() makes it possible for a PCRE2 client to
       find the value of certain configuration parameters and to discover
       which optional features have been compiled into the PCRE2 library. The
       pcre2build documentation has more details about these features.

       The first argument for pcre2_config() specifies which information is
       required. The second argument is a pointer to memory into which the
       information is placed. If NULL is passed, the function returns the
       amount of memory that is needed for the requested information. For
       calls that return numerical values, the value is in bytes; when
       requesting these values, where should point to appropriately aligned
       memory. For calls that return strings, the required length is given in
       code units, not counting the terminating zero.

       When requesting information, the returned value from pcre2_config() is
       non-negative on success, or the negative error code
       PCRE2_ERROR_BADOPTION if the value in the first argument is not
       recognized. The following information is available:

           PCRE2_CONFIG_BSR        The output is a uint32_t integer whose
                                   value indicates what character sequences
                                   the \R escape sequence matches by default.
                                   A value of PCRE2_BSR_UNICODE means that \R
                                   matches any Unicode line ending sequence; a
                                   value of PCRE2_BSR_ANYCRLF means that \R
                                   matches only CR, LF, or CRLF. The default
                                   can be overridden when a pattern is
                                   compiled.
           PCRE2_CONFIG_COMPILED_WIDTHS
                                   The output is a uint32_t integer whose
                                   lower bits indicate which code unit widths
                                   were selected when PCRE2 was built. The
                                   1-bit indicates 8-bit support, and the
                                   2-bit and 4-bit indicate 16-bit and 32-bit
                                   support, respectively.
           PCRE2_CONFIG_DEPTHLIMIT The output is a uint32_t integer that gives
                                   the default limit for the depth of nested
                                   backtracking in pcre2_match() or the depth
                                   of nested recursions, lookarounds, and
                                   atomic groups in pcre2_dfa_match(). Further
                                   details are given with
                                   pcre2_set_depth_limit() above.
           PCRE2_CONFIG_EFFECTIVE_LINKSIZE
                                   The output is a uint32_t integer that
                                   contains the number of bytes the library
                                   uses for internal linkage in compiled
                                   regular expressions. Its value is derived
                                   from the value that was provided at build
                                   time and that is described below by
                                   PCRE2_CONFIG_LINKSIZE.
           PCRE2_CONFIG_HEAPLIMIT  The output is a uint32_t integer that
                                   gives, in kibibytes, the default limit for
                                   the amount of heap memory used by
                                   pcre2_match() or pcre2_dfa_match(). Further
                                   details are given with
                                   pcre2_set_heap_limit() above.
           PCRE2_CONFIG_JIT        The output is a uint32_t integer that is
                                   set to one if support for just-in-time
                                   compiling is included in the library;
                                   otherwise it is set to zero. Note that
                                   having the support in the library does not
                                   guarantee that JIT will be used for any
                                   given match, and neither does it guarantee
                                   that JIT will actually be able to function,
                                   because it may not be able to allocate
                                   executable memory in some environments.
                                   There is a special call to
                                   pcre2_jit_compile() that can be used to
                                   check this. See the pcre2jit documentation
                                   for more details.
           PCRE2_CONFIG_JITTARGET  The where argument should point to a
                                   code-unit-aligned buffer. All previous
                                   versions of PCRE2 have required no more
                                   than 128 code units of buffer capacity.
                                   However, this requirement is not guaranteed
                                   to be maintained, so applications should
                                   call pcre2_config() with where set to NULL
                                   to receive the required buffer size, then
                                   assert or allocate a suitably-size buffer
                                   for a second call to pcre2_config(). The
                                   buffer is filled with a string that
                                   contains the name of the architecture for
                                   which the JIT compiler is configured at
                                   build time, for example, a 64-bit ARM CPU
                                   that supports the Armv8.1 extension writes
                                   "ARM-64 (LSE) 64bit (little endian +
                                   unaligned)". If JIT support is not
                                   available, PCRE2_ERROR_BADOPTION is
                                   returned; otherwise the number of code
                                   units used is returned. This is the length
                                   of the string plus one unit for the
                                   terminating zero.
           PCRE2_CONFIG_LINKSIZE   The output is a uint32_t integer that
                                   contains the number of bytes the library
                                   was instructed to use for internal linkage
                                   in compiled regular expressions. When PCRE2
                                   is configured, the value can be set to 2,
                                   3, or 4, with the default being 2 for most
                                   libraries.

                                   The actual number of bytes used depends on
                                   the size of the code units that the library
                                   supports and can be higher. See
                                   PCRE2_CONFIG_EFFECTIVE_LINKSIZE above for
                                   details.

                                   The default value of 2 for the 8-bit and
                                   16-bit libraries is sufficient for all but
                                   the most massive patterns, since it allows
                                   the size of the compiled pattern to be up
                                   to 65535 code units. Larger values allow
                                   larger regular expressions to be compiled
                                   by those two libraries, but at the expense
                                   of slower matching.
           PCRE2_CONFIG_MATCHLIMIT The output is a uint32_t integer that gives
                                   the default match limit for pcre2_match().
                                   Further details are given with
                                   pcre2_set_match_limit() above.
           PCRE2_CONFIG_NEWLINE    The output is a uint32_t integer whose
                                   value specifies the default character
                                   sequence that is recognized as meaning
                                   "newline". The values are:

         PCRE2_NEWLINE_CR       Carriage return (CR)
         PCRE2_NEWLINE_LF       Linefeed (LF)
         PCRE2_NEWLINE_CRLF     Carriage return, linefeed (CRLF)
         PCRE2_NEWLINE_ANY      Any Unicode line ending
         PCRE2_NEWLINE_ANYCRLF  Any of CR, LF, or CRLF
         PCRE2_NEWLINE_NUL      The NUL character (binary zero)

       The default should normally correspond to the standard sequence for
       your operating system.

       PCRE2_CONFIG_NEVER_BACKSLASH_C
                               The output is a uint32_t integer that is set to
                               one if the use of \C was permanently disabled
                               when PCRE2 was built; otherwise it is set to
                               zero.

       PCRE2_CONFIG_PARENSLIMIT
                               The output is a uint32_t integer that gives the
                               maximum depth of nesting of parentheses (of any
                               kind) in a pattern. This limit is imposed to
                               cap the amount of system stack used when a
                               pattern is compiled. It is specified when PCRE2
                               is built; the default is 250. This limit does
                               not take into account the stack that may
                               already be used by the calling application. For
                               finer control over compilation stack usage, see
                               pcre2_set_compile_recursion_guard().

       PCRE2_CONFIG_STACKRECURSE
                               This parameter is obsolete and should not be
                               used in new code. The output is a uint32_t
                               integer that is always set to zero.

       PCRE2_CONFIG_TABLES_LENGTH
                               The output is a uint32_t integer that gives the
                               length of PCRE2's character processing tables
                               in bytes. For details of these tables see the
                               section on locale support below.

       PCRE2_CONFIG_UNICODE_VERSION
                               The where argument should point to a
                               code-unit-aligned buffer. All previous versions
                               of PCRE2 have required no more than 24 code
                               units of buffer capacity. However, applications
                               should call pcre2_config() with where set to
                               NULL to receive the required buffer size, then
                               assert or allocate a suitably-size buffer for a
                               second call to pcre2_config(). If PCRE2 has
                               been compiled without Unicode support, the
                               buffer is filled with the text "Unicode not
                               supported". Otherwise, the Unicode version
                               string (for example, "8.0.0") is written. The
                               number of code units used is returned. This is
                               the length of the string plus one unit for the
                               terminating zero.

       PCRE2_CONFIG_UNICODE    The output is a uint32_t integer that is set to
                               one if Unicode support is available; otherwise
                               it is set to zero. Unicode support implies UTF
                               support.

       PCRE2_CONFIG_VERSION    The where argument should point to a
                               code-unit-aligned buffer. All previous versions
                               of PCRE2 have required no more than 24 code
                               units of buffer capacity. However, applications
                               should call pcre2_config() with where set to
                               NULL to receive the required buffer size, then
                               assert or allocate a suitably-size buffer for a
                               second call to pcre2_config(). The buffer is
                               filled with the PCRE2 version string,
                               zero-terminated. The number of code units used
                               is returned. This is the length of the string
                               plus one unit for the terminating zero.


COMPILING A PATTERN
       pcre2_code *pcre2_compile(PCRE2_SPTR pattern, PCRE2_SIZE patlen,
                                 uint32_t options, int *errorptr,
                                 PCRE2_SIZE *erroroffset,
                                 pcre2_compile_context *ccontext);

       void pcre2_code_free(pcre2_code *code);

       pcre2_code *pcre2_code_copy(const pcre2_code *code);

       pcre2_code *pcre2_code_copy_with_tables(const pcre2_code *code);

       The pcre2_compile() function compiles a pattern into an internal form.
       The pattern is defined by a pointer to a string of code units and a
       length in code units. If the pattern is zero-terminated, the length can
       be specified as PCRE2_ZERO_TERMINATED. A NULL pattern pointer with a
       length of zero is treated as an empty string (NULL with a non-zero
       length causes an error return). The function returns a pointer to a
       block of memory that contains the compiled pattern and related data, or
       NULL if an error occurred.

       If the compile context argument ccontext is NULL, memory for the
       compiled pattern is obtained by calling malloc(). Otherwise, it is
       obtained from the same memory function that was used for the compile
       context. The caller must free the memory by calling pcre2_code_free()
       when it is no longer needed. If pcre2_code_free() is called with a NULL
       argument, it returns immediately, without doing anything.

       The function pcre2_code_copy() makes a copy of the compiled code in new
       memory, using the same memory allocator as was used for the original.
       However, if the code has been processed by the JIT compiler (see
       below), the JIT information cannot be copied (because it is
       position-dependent). The new copy can initially be used only for
       non-JIT matching, though it can be passed to pcre2_jit_compile() if
       required. If pcre2_code_copy() is called with a NULL argument, it
       returns NULL.

       The pcre2_code_copy() function provides a way for individual threads in
       a multithreaded application to acquire a private copy of shared
       compiled code. However, it does not make a copy of the character tables
       used by the compiled pattern; the new pattern code points to the same
       tables as the original code. (See "Locale Support" below for details of
       these character tables.) In many applications the same tables are used
       throughout, so this behaviour is appropriate. Nevertheless, there are
       occasions when a copy of a compiled pattern and the relevant tables are
       needed. The pcre2_code_copy_with_tables() provides this facility.
       Copies of both the code and the tables are made, with the new code
       pointing to the new tables. The memory for the new tables is
       automatically freed when pcre2_code_free() is called for the new copy
       of the compiled code. If pcre2_code_copy_with_tables() is called with a
       NULL argument, it returns NULL.

           Note

           When one of the matching functions is called, pointers to the
           compiled pattern and the subject string are set in the match data
           block so that they can be referenced by the substring extraction
           functions after a successful match. After running a match, you must
           not free a compiled pattern or a subject string until after all
           operations on the match data block have taken place, unless, in the
           case of the subject string, you have used the
           PCRE2_COPY_MATCHED_SUBJECT option, which is described in the
           section entitled "Option bits for pcre2_match()" below.

       The options argument for pcre2_compile() contains various bit settings
       that affect the compilation. It should be zero if none of them are
       required. The available options are described below. Some of them (in
       particular, those that are compatible with Perl, but some others as
       well) can also be set and unset from within the pattern (see the
       detailed description in the pcre2pattern documentation).

       For those options that can be different in different parts of the
       pattern, the contents of the options argument specifies their settings
       at the start of compilation. The PCRE2_ANCHORED, PCRE2_ENDANCHORED, and
       PCRE2_NO_UTF_CHECK options can be set at the time of matching as well
       as at compile time.

       Some additional options and less frequently required compile-time
       parameters (for example, the newline setting) can be provided in a
       compile context (as described above).

       If errorcode or erroroffset is NULL, pcre2_compile() returns NULL
       immediately. Otherwise, the variables to which these point are set to
       an error code and an offset (number of code units) within the pattern,
       respectively, when pcre2_compile() returns NULL because a compilation
       error has occurred.

       There are over 100 positive error codes that pcre2_compile() may return
       if it finds an error in the pattern. There are also some negative error
       codes that are used for invalid UTF strings when validity checking is
       in force. These are the same as given by pcre2_match() and
       pcre2_dfa_match(), and are described in the pcre2unicode documentation.
       There is no separate documentation for the positive error codes,
       because the textual error messages that are obtained by calling the
       pcre2_get_error_message() function (see "Obtaining a textual error
       message" below) should be self-explanatory. Macro names starting with
       PCRE2_ERROR_ are defined for both positive and negative error codes in
       pcre2.h. When compilation is successful errorcode is set to a value
       that returns the message "no error" if passed to
       pcre2_get_error_message().

       The value returned in erroroffset is an indication of where in the
       pattern an error occurred. When there is no error, zero is returned. A
       non-zero value is not necessarily the furthest point in the pattern
       that was read. For example, after the error "lookbehind assertion is
       not fixed length", the error offset points to the start of the failing
       assertion. For an invalid UTF-8 or UTF-16 string, the offset is that of
       the first code unit of the failing character.

       Some errors are not detected until the whole pattern has been scanned;
       in these cases, the offset passed back is the length of the pattern.
       Note that the offset is in code units, not characters, even in a UTF
       mode. It may sometimes point into the middle of a UTF-8 or UTF-16
       character.

       This code fragment shows a typical straightforward call to
       pcre2_compile():

       pcre2_code *re;
       PCRE2_SIZE erroffset;
       int errorcode;
       re = pcre2_compile(
         "^A.*Z",                /* the pattern */
         PCRE2_ZERO_TERMINATED,  /* the pattern is zero-terminated */
         0,                      /* default options */
         &errorcode,             /* for error code */
         &erroffset,             /* for error offset */
         NULL);                  /* no compile context */

   Main compile options
       The following names for option bits are defined in the pcre2.h header
       file:

           PCRE2_ANCHORED          If this bit is set, the pattern is forced
                                   to be "anchored", that is, it is
                                   constrained to match only at the first
                                   matching point in the string that is being
                                   searched (the "subject string"). This
                                   effect can also be achieved by appropriate
                                   constructs in the pattern itself, which is
                                   the only way to do it in Perl.
           PCRE2_ALLOW_EMPTY_CLASS By default, for compatibility with Perl, a
                                   closing square bracket that immediately
                                   follows an opening one is treated as a data
                                   character for the class. When
                                   PCRE2_ALLOW_EMPTY_CLASS is set, it
                                   terminates the class, which therefore
                                   contains no characters and so can never
                                   match.
           PCRE2_ALT_BSUX          This option request alternative handling of
                                   three escape sequences, which makes PCRE2's
                                   behaviour more like ECMAscript (aka
                                   JavaScript). When it is set:

       (1) \U matches an upper case "U" character; by default \U causes a
       compile time error (Perl uses \U to upper case subsequent characters).

       (2) \u matches a lower case "u" character unless it is followed by four
       hexadecimal digits, in which case the hexadecimal number defines the
       code point to match. By default, \u causes a compile time error (Perl
       uses it to upper case the following character).

       (3) \x matches a lower case "x" character unless it is followed by two
       hexadecimal digits, in which case the hexadecimal number defines the
       code point to match. By default, as in Perl, a hexadecimal number is
       always expected after \x, but it may have one or two digits.

       ECMAscript 6 added additional functionality to \u. This can be accessed
       using the PCRE2_EXTRA_ALT_BSUX extra option (see "Extra compile
       options" below). Note that this alternative escape handling applies
       only to patterns. Neither of these options affects the processing of
       replacement strings passed to pcre2_substitute().

       PCRE2_ALT_CIRCUMFLEX    In multiline mode (when PCRE2_MULTILINE is
                               set), the circumflex metacharacter matches at
                               the start of the subject (unless PCRE2_NOTBOL
                               is set), and also after any internal newline.
                               However, it does not match after a newline at
                               the end of the subject, for compatibility with
                               Perl. If you want a multiline circumflex also
                               to match after a terminating newline, you must
                               set PCRE2_ALT_CIRCUMFLEX.

       PCRE2_ALT_EXTENDED_CLASS
                               Alters the parsing of character classes to
                               follow the extended syntax described by Unicode
                               UTS#18. The PCRE2_ALT_EXTENDED_CLASS option has
                               no impact on the behaviour of the Perl-specific
                               "(?[...])" syntax for extended classes, but
                               instead enables the alternative syntax of
                               extended class behaviour inside ordinary
                               "[...]" character classes. See the pcre2pattern
                               documentation for details of the character
                               classes supported.

       PCRE2_ALT_VERBNAMES     By default, for compatibility with Perl, the
                               name in any verb sequence such as (*MARK:NAME)
                               is any sequence of characters that does not
                               include a closing parenthesis. The name is not
                               processed in any way, and it is not possible to
                               include a closing parenthesis in the name.
                               However, if the PCRE2_ALT_VERBNAMES option is
                               set, normal backslash processing is applied to
                               verb names and only an unescaped closing
                               parenthesis terminates the name. A closing
                               parenthesis can be included in a name either as
                               \) or between \Q and \E. If the PCRE2_EXTENDED
                               or PCRE2_EXTENDED_MORE option is set with
                               PCRE2_ALT_VERBNAMES, unescaped white space in
                               verb names is skipped and #-comments are
                               recognized, exactly as in the rest of the
                               pattern.

       PCRE2_AUTO_CALLOUT      If this bit is set, pcre2_compile()
                               automatically inserts callout items, all with
                               number 255, before each pattern item, except
                               immediately before or after an explicit callout
                               in the pattern. For discussion of the callout
                               facility, see the pcre2callout documentation.

       PCRE2_CASELESS          If this bit is set, letters in the pattern
                               match both upper and lower case letters in the
                               subject. It is equivalent to Perl's /i option,
                               and it can be changed within a pattern by a
                               (?i) option setting. If either PCRE2_UTF or
                               PCRE2_UCP is set, Unicode properties are used
                               for all characters with more than one other
                               case, and for all characters whose code points
                               are greater than U+007F.

       Note that there are two ASCII characters, K and S, that, in addition to
       their lower case ASCII equivalents, are case-equivalent with U+212A
       (Kelvin sign) and U+017F (long S) respectively. If you do not want this
       case equivalence, you can suppress it by setting
       PCRE2_EXTRA_CASELESS_RESTRICT.

       One language family, Turkish and Azeri, has its own case-insensitivity
       rules, which can be selected by setting PCRE2_EXTRA_TURKISH_CASING.
       This alters the behaviour of the 'i', 'I', U+0130 (capital I with dot
       above), and U+0131 (small dotless i) characters.

       For lower valued characters with only one other case, a lookup table is
       used for speed. When neither PCRE2_UTF nor PCRE2_UCP is set, a lookup
       table is used for all code points less than 256, and higher code points
       (available only in 16-bit or 32-bit mode) are treated as not having
       another case.

       From release 10.45 PCRE2_CASELESS also affects what some of the
       letter-related Unicode property escapes (\p and \P) match. The
       properties Lu (upper case letter), Ll (lower case letter), and Lt
       (title case letter) are all treated as LC (cased letter) when
       PCRE2_CASELESS is set.

       PCRE2_DOLLAR_ENDONLY If this bit is set, a dollar metacharacter in the
                            pattern matches only at the end of the subject
                            string. Without this option, a dollar also matches
                            immediately before a newline at the end of the
                            string (but not before any other newlines). The
                            PCRE2_DOLLAR_ENDONLY option is ignored if
                            PCRE2_MULTILINE is set. There is no equivalent to
                            this option in Perl, and no way to set it within a
                            pattern.

       PCRE2_DOTALL         If this bit is set, a dot metacharacter in the
                            pattern matches any character, including one that
                            indicates a newline. However, it only ever matches
                            one character, even if newlines are coded as CRLF.
                            Without this option, a dot does not match when the
                            current position in the subject is at a newline.
                            This option is equivalent to Perl's /s option, and
                            it can be changed within a pattern by a (?s)
                            option setting. A negative class such as [^a]
                            always matches newline characters, and the \N
                            escape sequence always matches a non-newline
                            character, independent of the setting of
                            PCRE2_DOTALL.

       PCRE2_DUPNAMES       If this bit is set, names used to identify capture
                            groups need not be unique. This can be helpful for
                            certain types of pattern when it is known that
                            only one instance of the named group can ever be
                            matched. There are more details of named capture
                            groups below; see also the pcre2pattern
                            documentation.

       PCRE2_ENDANCHORED    If this bit is set, the end of any pattern match
                            must be right at the end of the string being
                            searched (the "subject string"). If the pattern
                            match succeeds by reaching (*ACCEPT), but does not
                            reach the end of the subject, the match fails at
                            the current starting point. For unanchored
                            patterns, a new match is then tried at the next
                            starting point. However, if the match succeeds by
                            reaching the end of the pattern, but not the end
                            of the subject, backtracking occurs and an
                            alternative match may be found. Consider these two
                            patterns:

       .(*ACCEPT)|..
       .|..

       If matched against "abc" with PCRE2_ENDANCHORED set, the first matches
       "c" whereas the second matches "bc". The effect of PCRE2_ENDANCHORED
       can also be achieved by appropriate constructs in the pattern itself,
       which is the only way to do it in Perl.

       For DFA matching with pcre2_dfa_match(), PCRE2_ENDANCHORED applies only
       to the first (that is, the longest) matched string. Other parallel
       matches, which are necessarily substrings of the first one, must
       obviously end before the end of the subject.

       PCRE2_EXTENDED If this bit is set, most white space characters in the
                      pattern are totally ignored except when escaped, inside
                      a character class, or inside a \Q...\E sequence.
                      However, white space is not allowed within sequences
                      such as (?> that introduce various parenthesized groups,
                      nor within numerical quantifiers such as {1,3}.
                      Ignorable white space is permitted between an item and a
                      following quantifier and between a quantifier and a
                      following + that indicates possessiveness.
                      PCRE2_EXTENDED is equivalent to Perl's /x option, and it
                      can be changed within a pattern by a (?x) option
                      setting.

       When PCRE2 is compiled without Unicode support, PCRE2_EXTENDED
       recognizes as white space only those characters with code points less
       than 256 that are flagged as white space in its low-character table.
       The table is normally created by pcre2_maketables(), which uses the
       isspace() function to identify space characters. In most ASCII
       environments, the relevant characters are those with code points 0x0009
       (tab), 0x000A (linefeed), 0x000B (vertical tab), 0x000C (formfeed),
       0x000D (carriage return), and 0x0020 (space).

       When PCRE2 is compiled with Unicode support, in addition to these
       characters, five more Unicode "Pattern White Space" characters are
       recognized by PCRE2_EXTENDED. These are U+0085 (next line), U+200E
       (left-to-right mark), U+200F (right-to-left mark), U+2028 (line
       separator), and U+2029 (paragraph separator). This set of characters is
       the same as recognized by Perl's /x option. Note that the horizontal
       and vertical space characters that are matched by the \h and \v escapes
       in patterns are a much bigger set.

       As well as ignoring most white space, PCRE2_EXTENDED also causes
       characters between an unescaped # outside a character class and the
       next newline, inclusive, to be ignored, which makes it possible to
       include comments inside complicated patterns. Note that the end of this
       type of comment is a literal newline sequence in the pattern; escape
       sequences that happen to represent a newline do not count.

       Which characters are interpreted as newlines can be specified by a
       setting in the compile context that is passed to pcre2_compile() or by
       a special sequence at the start of the pattern, as described in the
       section entitled "Newline conventions" in the pcre2pattern
       documentation. A default is defined when PCRE2 is built.

       PCRE2_EXTENDED_MORE     This option has the effect of PCRE2_EXTENDED,
                               but, in addition, unescaped space and
                               horizontal tab characters are ignored inside a
                               character class. Note: only these two
                               characters are ignored, not the full set of
                               pattern white space characters that are ignored
                               outside a character class. PCRE2_EXTENDED_MORE
                               is equivalent to Perl's /xx option, and it can
                               be changed within a pattern by a (?xx) option
                               setting.

       PCRE2_FIRSTLINE         If this option is set, the start of an
                               unanchored pattern match must be before or at
                               the first newline in the subject string
                               following the start of matching, though the
                               matched text may continue over the newline. If
                               start_offset is non-zero, the limiting newline
                               is not necessarily the first newline in the
                               subject. For example, if the subject string is
                               "abc\nxyz" (where \n represents a
                               single-character newline) a pattern match for
                               "yz" succeeds with PCRE2_FIRSTLINE if
                               start_offset is greater than 3. See also
                               PCRE2_USE_OFFSET_LIMIT, which provides a more
                               general limiting facility. If PCRE2_FIRSTLINE
                               is set with an offset limit, a match must occur
                               in the first line and also within the offset
                               limit. In other words, whichever limit comes
                               first is used. This option has no effect for
                               anchored patterns.

       PCRE2_LITERAL           If this option is set, all meta-characters in
                               the pattern are disabled, and it is treated as
                               a literal string. Matching literal strings with
                               a regular expression engine is not the most
                               efficient way of doing it. If you are doing a
                               lot of literal matching and are worried about
                               efficiency, you should consider using other
                               approaches. The only other main options that
                               are allowed with PCRE2_LITERAL are:
                               PCRE2_ANCHORED, PCRE2_ENDANCHORED,
                               PCRE2_AUTO_CALLOUT, PCRE2_CASELESS,
                               PCRE2_FIRSTLINE, PCRE2_MATCH_INVALID_UTF,
                               PCRE2_NO_START_OPTIMIZE, PCRE2_NO_UTF_CHECK,
                               PCRE2_UTF, and PCRE2_USE_OFFSET_LIMIT. The
                               extra options PCRE2_EXTRA_MATCH_LINE and
                               PCRE2_EXTRA_MATCH_WORD are also supported. Any
                               other options cause an error.

       PCRE2_MATCH_INVALID_UTF This option forces PCRE2_UTF (see below) and
                               also enables support for matching by
                               pcre2_match() in subject strings that contain
                               invalid UTF sequences. Note, however, that the
                               16-bit and 32-bit PCRE2 libraries process
                               strings as sequences of uint16_t or uint32_t
                               code points. They cannot find valid UTF
                               sequences within an arbitrary string of bytes
                               unless such sequences are suitably aligned.
                               This facility is not supported for DFA
                               matching. For details, see the pcre2unicode
                               documentation.

       PCRE2_MATCH_UNSET_BACKREF
                               If this option is set, a backreference to an
                               unset capture group matches an empty string (by
                               default this causes the current matching
                               alternative to fail). A pattern such as (\1)(a)
                               succeeds when this option is set (assuming it
                               can find an "a" in the subject), whereas it
                               fails by default, for Perl compatibility.
                               Setting this option makes PCRE2 behave more
                               like ECMAscript (aka JavaScript).

       PCRE2_MULTILINE         By default, for the purposes of matching "start
                               of line" and "end of line", PCRE2 treats the
                               subject string as consisting of a single line
                               of characters, even if it actually contains
                               newlines. The "start of line" metacharacter (^)
                               matches only at the start of the string, and
                               the "end of line" metacharacter ($) matches
                               only at the end of the string, or before a
                               terminating newline (except when
                               PCRE2_DOLLAR_ENDONLY is set). Note, however,
                               that unless PCRE2_DOTALL is set, the "any
                               character" metacharacter (.) does not match at
                               a newline. This behaviour (for ^, $, and dot)
                               is the same as Perl.

       When PCRE2_MULTILINE it is set, the "start of line" and "end of line"
       constructs match immediately following or immediately before internal
       newlines in the subject string, respectively, as well as at the very
       start and end. This is equivalent to Perl's /m option, and it can be
       changed within a pattern by a (?m) option setting. Note that the "start
       of line" metacharacter does not match after a newline at the end of the
       subject, for compatibility with Perl. However, you can change this by
       setting the PCRE2_ALT_CIRCUMFLEX option. If there are no newlines in a
       subject string, or no occurrences of ^ or $ in a pattern, setting
       PCRE2_MULTILINE has no effect.

       PCRE2_NEVER_BACKSLASH_C This option locks out the use of \C in the
                               pattern that is being compiled. This escape can
                               cause unpredictable behaviour in UTF-8 or
                               UTF-16 modes, because it may leave the current
                               matching point in the middle of a
                               multi-code-unit character. This option may be
                               useful in applications that process patterns
                               from external sources. Note that there is also
                               a build-time option that permanently locks out
                               the use of \C.

       PCRE2_NEVER_UCP         This option locks out the use of Unicode
                               properties for handling \B, \b, \D, \d, \S, \s,
                               \W, \w, and some of the POSIX character
                               classes, as described for the PCRE2_UCP option
                               below. In particular, it prevents the creator
                               of the pattern from enabling this facility by
                               starting the pattern with (*UCP). This option
                               may be useful in applications that process
                               patterns from external sources. The option
                               combination PCRE2_UCP and PCRE2_NEVER_UCP
                               causes an error.

       PCRE2_NEVER_UTF         This option locks out interpretation of the
                               pattern as UTF-8, UTF-16, or UTF-32, depending
                               on which library is in use. In particular, it
                               prevents the creator of the pattern from
                               switching to UTF interpretation by starting the
                               pattern with (*UTF). This option may be useful
                               in applications that process patterns from
                               external sources. The combination of PCRE2_UTF
                               and PCRE2_NEVER_UTF causes an error.

       PCRE2_NO_AUTO_CAPTURE   If this option is set, it disables the use of
                               numbered capturing parentheses in the pattern.
                               Any opening parenthesis that is not followed by
                               ? behaves as if it were followed by ?: but
                               named parentheses can still be used for
                               capturing (and they acquire numbers in the
                               usual way). This is the same as Perl's /n
                               option. Note that, when this option is set,
                               references to capture groups (backreferences or
                               recursion/subroutine calls) may only refer to
                               named groups, though the reference can be by
                               name or by number.

       PCRE2_NO_AUTO_POSSESS   If this (deprecated) option is set, it disables
                               "auto-possessification", which is an
                               optimization that, for example, turns a+b into
                               a++b in order to avoid backtracks into a++ that
                               can never be successful. However, if callouts
                               are in use, auto-possessification means that
                               some callouts are never taken. You can set this
                               option if you want the matching functions to do
                               a full unoptimized search and run all the
                               callouts, but it is mainly provided for testing
                               purposes.

       If a compile context is available, it is recommended to use
       pcre2_set_optimize() with the directive PCRE2_AUTO_POSSESS_OFF rather
       than the compile option PCRE2_NO_AUTO_POSSESS. Note that
       PCRE2_NO_AUTO_POSSESS takes precedence over the pcre2_set_optimize()
       optimization directives PCRE2_AUTO_POSSESS and PCRE2_AUTO_POSSESS_OFF.

       PCRE2_NO_DOTSTAR_ANCHOR If this (deprecated) option is set, it disables
                               an optimization that is applied when . is the
                               first significant item in a top-level branch of
                               a pattern, and all the other branches also
                               start with . or with \A or \G or ^. The
                               optimization is automatically disabled for . if
                               it is inside an atomic group or a capture group
                               that is the subject of a backreference, or if
                               the pattern contains (*PRUNE) or (*SKIP). When
                               the optimization is not disabled, such a
                               pattern is automatically anchored if
                               PCRE2_DOTALL is set for all the . items and
                               PCRE2_MULTILINE is not set for any ^ items.
                               Otherwise, the fact that any match must start
                               either at the start of the subject or following
                               a newline is remembered. Like other
                               optimizations, this can cause callouts to be
                               skipped. (If a compile context is available, it
                               is recommended to use pcre2_set_optimize() with
                               the directive PCRE2_DOTSTAR_ANCHOR_OFF
                               instead.)

       PCRE2_NO_START_OPTIMIZE This is an option whose main effect is at
                               matching time. It does not change what
                               pcre2_compile() generates, but it does affect
                               the output of the JIT compiler. Setting this
                               option is equivalent to calling
                               pcre2_set_optimize() with the directive
                               parameter set to PCRE2_START_OPTIMIZE_OFF.

       There are a number of optimizations that may occur at the start of a
       match, in order to speed up the process. For example, if it is known
       that an unanchored match must start with a specific code unit value,
       the matching code searches the subject for that value, and fails
       immediately if it cannot find it, without actually running the main
       matching function. The start-up optimizations are in effect a pre-scan
       of the subject that takes place before the pattern is run.

       Disabling the start-up optimizations may cause performance to suffer.
       However, this may be desirable for patterns which contain callouts or
       items such as (*COMMIT) and (*MARK). See the above description of
       PCRE2_START_OPTIMIZE_OFF for further details.

       PCRE2_NO_UTF_CHECK When PCRE2_UTF is set, the validity of the pattern
                          as a UTF string is automatically checked. There are
                          discussions about the validity of UTF-8 strings,
                          UTF-16 strings, and UTF-32 strings in the
                          pcre2unicode document. If an invalid UTF sequence is
                          found, pcre2_compile() returns a negative error
                          code.

       If you know that your pattern is a valid UTF string, and you want to
       skip this check for performance reasons, you can set the
       PCRE2_NO_UTF_CHECK option. When it is set, the effect of passing an
       invalid UTF string as a pattern is undefined. It may cause your program
       to crash or loop.

       Note that this option can also be passed to pcre2_match() and
       pcre2_dfa_match(), to suppress UTF validity checking of the subject
       string.

       Note also that setting PCRE2_NO_UTF_CHECK at compile time does not
       disable the error that is given if an escape sequence for an invalid
       Unicode code point is encountered in the pattern. In particular, the
       so-called "surrogate" code points (0xd800 to 0xdfff) are invalid. If
       you want to allow escape sequences such as \x{d800} you can set the
       PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES extra option, as described in the
       section entitled "Extra compile options" below. However, this is
       possible only in UTF-8 and UTF-32 modes, because these values are not
       representable in UTF-16.

       PCRE2_UCP This option has two effects. Firstly, it change the way PCRE2
                 processes \B, \b, \D, \d, \S, \s, \W, \w, and some of the
                 POSIX character classes. By default, only ASCII characters
                 are recognized, but if PCRE2_UCP is set, Unicode properties
                 are used to classify characters. There are some PCRE2_EXTRA
                 options (see below) that add finer control to this behaviour.
                 More details are given in the section on generic character
                 types in the pcre2pattern page.

       The second effect of PCRE2_UCP is to force the use of Unicode
       properties for upper/lower casing operations, even when PCRE2_UTF is
       not set. This makes it possible to process strings in the 16-bit UCS-2
       code. This option is available only if PCRE2 has been compiled with
       Unicode support (which is the default).

       The PCRE2_EXTRA_CASELESS_RESTRICT option (see above) restricts caseless
       matching such that ASCII characters match only ASCII characters and
       non-ASCII characters match only non-ASCII characters. The
       PCRE2_EXTRA_TURKISH_CASING option (see above) alters the matching of
       the 'i' characters to follow their behaviour in Turkish and Azeri
       languages. For further details on PCRE2_EXTRA_CASELESS_RESTRICT and
       PCRE2_EXTRA_TURKISH_CASING, see the pcre2unicode page.

       PCRE2_UNGREEDY         This option inverts the "greediness" of the
                              quantifiers so that they are not greedy by
                              default, but become greedy if followed by "?".
                              It is not compatible with Perl. It can also be
                              set by a (?U) option setting within the pattern.

       PCRE2_USE_OFFSET_LIMIT This option must be set for pcre2_compile() if
                              pcre2_set_offset_limit() is going to be used to
                              set a non-default offset limit in a match
                              context for matches that use this pattern. An
                              error is generated if an offset limit is set
                              without this option. For more details, see the
                              description of pcre2_set_offset_limit() in the
                              section that describes match contexts. See also
                              the PCRE2_FIRSTLINE option above.

       PCRE2_UTF              This option causes PCRE2 to regard both the
                              pattern and the subject strings that are
                              subsequently processed as strings of UTF
                              characters instead of single-code-unit strings.
                              It is available when PCRE2 is built to include
                              Unicode support (which is the default). If
                              Unicode support is not available, the use of
                              this option provokes an error. Details of how
                              PCRE2_UTF changes the behaviour of PCRE2 are
                              given in the pcre2unicode page. In particular,
                              note that it changes the way PCRE2_CASELESS
                              works.

   Extra compile options
       The option bits that can be set in a compile context by calling the
       pcre2_set_compile_extra_options() function are as follows:

           PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK

                                   Since release 10.38 PCRE2 has forbidden the
                                   use of \K within lookaround assertions,
                                   following Perl's lead. This option is
                                   provided to re-enable the previous
                                   behaviour (act in positive lookarounds,
                                   ignore in negative ones) in case anybody is
                                   relying on it.

           PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES

                                   This option applies when compiling a
                                   pattern in UTF-8 or UTF-32 mode. It is
                                   forbidden in UTF-16 mode, and ignored in
                                   non-UTF modes. Unicode "surrogate" code
                                   points in the range 0xd800 to 0xdfff are
                                   used in pairs in UTF-16 to encode code
                                   points with values in the range 0x10000 to
                                   0x10ffff. The surrogates cannot therefore
                                   be represented in UTF-16. They can be
                                   represented in UTF-8 and UTF-32, but are
                                   defined as invalid code points, and cause
                                   errors if encountered in a UTF-8 or UTF-32
                                   string that is being checked for validity
                                   by PCRE2.

       These values also cause errors if encountered in escape sequences such
       as \x{d912} within a pattern. However, it seems that some applications,
       when using PCRE2 to check for unwanted characters in UTF-8 strings,
       explicitly test for the surrogates using escape sequences. The
       PCRE2_NO_UTF_CHECK option does not disable the error that occurs,
       because it applies only to the testing of input strings for UTF
       validity.

       If the extra option PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES is set,
       surrogate code point values in UTF-8 and UTF-32 patterns no longer
       provoke errors and are incorporated in the compiled pattern. However,
       they can only match subject characters if the matching function is
       called with PCRE2_NO_UTF_CHECK set.

           PCRE2_EXTRA_ALT_BSUX

                                The original option PCRE2_ALT_BSUX causes
                                PCRE2 to process \U, \u, and \x in the way
                                that ECMAscript (aka JavaScript) does.
                                Additional functionality was defined by
                                ECMAscript 6; setting PCRE2_EXTRA_ALT_BSUX has
                                the effect of PCRE2_ALT_BSUX, but in addition
                                it recognizes \u{hhh..} as a hexadecimal
                                character code, where hhh.. is any number of
                                hexadecimal digits.

           PCRE2_EXTRA_ASCII_BSD

                                 This option forces \d to match only ASCII
                                 digits, even when PCRE2_UCP is set. It can be
                                 changed within a pattern by means of the
                                 (?aD) option setting.

           PCRE2_EXTRA_ASCII_BSS

                                 This option forces \s to match only ASCII
                                 space characters, even when PCRE2_UCP is set.
                                 It can be changed within a pattern by means
                                 of the (?aS) option setting.

           PCRE2_EXTRA_ASCII_BSW

                                 This option forces \w to match only ASCII
                                 word characters, even when PCRE2_UCP is set.
                                 It can be changed within a pattern by means
                                 of the (?aW) option setting.

           PCRE2_EXTRA_ASCII_DIGIT

                                   This option forces the POSIX character
                                   classes [:digit:] and [:xdigit:] to match
                                   only ASCII digits, even when PCRE2_UCP is
                                   set. It can be changed within a pattern by
                                   means of the (?aT) option setting.

           PCRE2_EXTRA_ASCII_POSIX

                                   This option forces all the POSIX character
                                   classes, including [:digit:] and
                                   [:xdigit:], to match only ASCII characters,
                                   even when PCRE2_UCP is set. It can be
                                   changed within a pattern by means of the
                                   (?aP) option setting, but note that this
                                   also sets PCRE2_EXTRA_ASCII_DIGIT in order
                                   to ensure that (?-aP) unsets all ASCII
                                   restrictions for POSIX classes.

           PCRE2_EXTRA_BAD_ESCAPE_IS_LITERAL

                                   This is a dangerous option. Use with care.
                                   By default, an unrecognized escape such as
                                   \j or a malformed one such as \x{2z} causes
                                   a compile-time error when detected by
                                   pcre2_compile(). Perl is somewhat
                                   inconsistent in handling such items: for
                                   example, \j is treated as a literal "j",
                                   and non-hexadecimal digits in \x{} are just
                                   ignored, though warnings are given in both
                                   cases if Perl's warning switch is enabled.
                                   However, a malformed octal number after \o{
                                   always causes an error in Perl.

       If the PCRE2_EXTRA_BAD_ESCAPE_IS_LITERAL extra option is passed to
       pcre2_compile(), all unrecognized or malformed escape sequences are
       treated as single-character escapes. For example, \j is a literal "j"
       and \x{2z} is treated as the literal string "x{2z}". Setting this
       option means that typos in patterns may go undetected and have
       unexpected results. Also note that a sequence such as [\N{] is
       interpreted as a malformed attempt at [\N{...] and so is treated as
       [N{] whereas [\N] gives an error because an unqualified \N is a valid
       escape sequence but is not supported in a character class. To
       reiterate: this is a dangerous option. Use with great care.

           PCRE2_EXTRA_CASELESS_RESTRICT

                                   When either PCRE2_UCP or PCRE2_UTF is set,
                                   caseless matching follows Unicode rules,
                                   which allow for more than two cases per
                                   character. There are two case-equivalent
                                   character sets that contain both ASCII and
                                   non-ASCII characters. The ASCII letter S is
                                   case-equivalent to U+017f (long S) and the
                                   ASCII letter K is case-equivalent to U+212a
                                   (Kelvin sign). This option disables
                                   recognition of case-equivalences that cross
                                   the ASCII/non-ASCII boundary. In a caseless
                                   match, both characters must either be ASCII
                                   or non-ASCII. The option can be changed
                                   within a pattern by the
                                   (*CASELESS_RESTRICT) or (?r) option
                                   settings.

           PCRE2_EXTRA_ESCAPED_CR_IS_LF

                                   There are some legacy applications where
                                   the escape sequence \r in a pattern is
                                   expected to match a newline. If this option
                                   is set, \r in a pattern is converted to \n
                                   so that it matches a LF (linefeed) instead
                                   of a CR (carriage return) character. The
                                   option does not affect a literal CR in the
                                   pattern, nor does it affect CR specified as
                                   an explicit code point such as \x{0D}.

           PCRE2_EXTRA_MATCH_LINE

                                  This option is provided for use by the -x
                                  option of pcre2grep. It causes the pattern
                                  only to match complete lines. This is
                                  achieved by automatically inserting the code
                                  ^(?: at the start of the compiled pattern
                                  and )$ at the end. Thus, when
                                  PCRE2_MULTILINE is set, the matched line may
                                  be in the middle of the subject string. This
                                  option can be used with PCRE2_LITERAL.

           PCRE2_EXTRA_MATCH_WORD

                                  This option is provided for use by the -w
                                  option of pcre2grep. It causes the pattern
                                  only to match strings that have a word
                                  boundary at the start and the end. This is
                                  achieved by automatically inserting the code
                                  for \b(?: at the start of the compiled
                                  pattern and )\b at the end. The option may
                                  be used with PCRE2_LITERAL. However, it is
                                  ignored if PCRE2_EXTRA_MATCH_LINE is also
                                  set.

           PCRE2_EXTRA_NO_BS0

                              If this option is set (note that its final
                              character is the digit 0) it locks out the use
                              of the sequence \0 unless at least one more
                              octal digit follows.

           PCRE2_EXTRA_PYTHON_OCTAL

                                   If this option is set, PCRE2 follows
                                   Python's rules for interpreting octal
                                   escape sequences. The rules for handling
                                   sequences such as \14, which could be an
                                   octal number or a back reference are
                                   different. Details are given in the
                                   pcre2pattern documentation.

           PCRE2_EXTRA_NEVER_CALLOUT

                                   If this option is set, PCRE2 treats
                                   callouts in the pattern as a syntax error,
                                   returning
                                   PCRE2_ERROR_CALLOUT_CALLER_DISABLED. This
                                   is useful if the application knows that a
                                   callout will not be provided to
                                   pcre2_match(), so that callouts in the
                                   pattern are not silently ignored.

           PCRE2_EXTRA_TURKISH_CASING

                                   This option alters case-equivalence of the
                                   'i' letters to follow the alphabet used by
                                   Turkish and Azeri languages. The option can
                                   be changed within a pattern by the
                                   (*TURKISH_CASING) start-of-pattern setting.
                                   Either the UTF or UCP options must be set.
                                   In the 8-bit library, UTF must be set. This
                                   option cannot be combined with
                                   PCRE2_EXTRA_CASELESS_RESTRICT.


JUST-IN-TIME (JIT) COMPILATION
       int pcre2_jit_compile(pcre2_code *code, uint32_t options);

       int pcre2_jit_match(const pcre2_code *code, PCRE2_SPTR subject,
                           PCRE2_SIZE length, PCRE2_SIZE start_offset,
                           uint32_t options, pcre2_match_data *match_data,
                           pcre2_match_context *mcontext);

       void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);

       pcre2_jit_stack *pcre2_jit_stack_create(
             size_t startsize, size_t maxsize,
             pcre2_general_context *gcontext);

       void pcre2_jit_stack_assign(pcre2_match_context *mcontext,
                                   pcre2_jit_callback callback,
                                   void *callback_data);

       void pcre2_jit_stack_free(pcre2_jit_stack *jit_stack);

       These functions provide support for JIT compilation, which, if the
       just-in-time compiler is available, further processes a compiled
       pattern into machine code that executes much faster than the
       pcre2_match() interpretive matching function. Full details are given in
       the pcre2jit documentation.

       JIT compilation is a heavyweight optimization. It can take some time
       for patterns to be analyzed, and for one-off matches and simple
       patterns the benefit of faster execution might be offset by a much
       slower compilation time. Most (but not all) patterns can be optimized
       by the JIT compiler.


LOCALE SUPPORT
       const uint8_t *pcre2_maketables(pcre2_general_context *gcontext);

       void pcre2_maketables_free(pcre2_general_context *gcontext,
                                  const uint8_t *tables);

       PCRE2 handles caseless matching, and determines whether characters are
       letters, digits, or whatever, by reference to a set of tables, indexed
       by character code point. However, this applies only to characters whose
       code points are less than 256. By default, higher-valued code points
       never match escapes such as \w or \d.

       When PCRE2 is built with Unicode support (the default), certain Unicode
       character properties can be tested with \p and \P, or, alternatively,
       the PCRE2_UCP option can be set when a pattern is compiled; this causes
       \w and friends to use Unicode property support instead of the built-in
       tables. PCRE2_UCP also causes upper/lower casing operations on
       characters with code points greater than 127 to use Unicode properties.
       These effects apply even when PCRE2_UTF is not set. There are, however,
       some PCRE2_EXTRA options (see above) that can be used to modify or
       suppress them.

       The use of locales with Unicode is discouraged. If you are handling
       characters with code points greater than 127, you should either use
       Unicode support, or use locales, but not try to mix the two.

       PCRE2 contains a built-in set of character tables that are used by
       default. These are sufficient for many applications. Normally, the
       internal tables recognize only ASCII characters. However, when PCRE2 is
       built, it is possible to cause the internal tables to be rebuilt in the
       default "C" locale of the local system, which may cause them to be
       different.

       The built-in tables can be overridden by tables supplied by the
       application that calls PCRE2. These may be created in a different
       locale from the default. As more and more applications change to using
       Unicode, the need for this locale support is expected to die away.

       External tables are built by calling the pcre2_maketables() function,
       in the relevant locale. The only argument to this function is a general
       context, which can be used to pass a custom memory allocator. If the
       argument is NULL, the system malloc() is used. The result can be passed
       to pcre2_compile() as often as necessary, by creating a compile context
       and calling pcre2_set_character_tables() to set the tables pointer
       therein.

       For example, to build and use tables that are appropriate for the
       French locale (where accented characters with values greater than 127
       are treated as letters), the following code could be used:

       setlocale(LC_CTYPE, "fr_FR");
       tables = pcre2_maketables(NULL);
       ccontext = pcre2_compile_context_create(NULL);
       pcre2_set_character_tables(ccontext, tables);
       re = pcre2_compile(..., ccontext);

       The locale name "fr_FR" is used on Linux and other Unix-like systems;
       if you are using Windows, the name for the French locale is "french".

       The pointer that is passed (via the compile context) to pcre2_compile()
       is saved with the compiled pattern, and the same tables are used by the
       matching functions. Thus, for any single pattern, compilation and
       matching both happen in the same locale, but different patterns can be
       processed in different locales.

       It is the caller's responsibility to ensure that the memory containing
       the tables remains available while they are still in use. When they are
       no longer needed, you can discard them using pcre2_maketables_free(),
       which should pass as its first parameter the same global context that
       was used to create the tables.

   Saving locale tables
       The tables described above are just a sequence of binary bytes, which
       makes them independent of hardware characteristics such as endianness
       or whether the processor is 32-bit or 64-bit. A copy of the result of
       pcre2_maketables() can therefore be saved in a file or elsewhere and
       re-used later, even in a different program or on another computer. The
       size of the tables (number of bytes) must be obtained by calling
       pcre2_config() with the PCRE2_CONFIG_TABLES_LENGTH option because
       pcre2_maketables() does not return this value. Note that the
       pcre2_dftables program, which is part of the PCRE2 build system, can be
       used stand-alone to create a file that contains a set of binary tables.
       See the pcre2build documentation for details.


INFORMATION ABOUT A COMPILED PATTERN
       int pcre2_pattern_info(const pcre2_code *code, uint32_t what,
                              void *where);

       The pcre2_pattern_info() function returns general information about a
       compiled pattern. For information about callouts, see the next section.
       The first argument for pcre2_pattern_info() is a pointer to the
       compiled pattern. The second argument specifies which piece of
       information is required, and the third argument is a pointer to a
       variable to receive the data. If the third argument is NULL, the first
       argument is ignored, and the function returns the size in bytes of the
       variable that is required for the information requested. Otherwise, the
       yield of the function is zero for success, or one of the following
       negative numbers:

           PCRE2_ERROR_NULL      the argument code was NULL
           PCRE2_ERROR_BADMAGIC  the "magic number" was not found
           PCRE2_ERROR_BADOPTION the value of what was invalid
           PCRE2_ERROR_UNSET     the requested field is not set

       The "magic number" is placed at the start of each compiled pattern as a
       simple check against passing an arbitrary memory pointer. Here is a
       typical call of pcre2_pattern_info(), to obtain the length of the
       compiled pattern:

       int rc;
       size_t length;
       rc = pcre2_pattern_info(
             re,               /* result of pcre2_compile() */
             PCRE2_INFO_SIZE,  /* what is required */
             &length);         /* where to put the data */

       The possible values for the second argument are defined in pcre2.h, and
       are as follows:

           PCRE2_INFO_ALLOPTIONS, PCRE2_INFO_ARGOPTIONS,
           PCRE2_INFO_EXTRAOPTIONS

         PCRE2_INFO_ALLOPTIONS
         PCRE2_INFO_ARGOPTIONS
         PCRE2_INFO_EXTRAOPTIONS

       Return copies of the pattern's options. The third argument should point
       to a uint32_t variable. PCRE2_INFO_ARGOPTIONS returns exactly the
       options that were passed to pcre2_compile(), whereas
       PCRE2_INFO_ALLOPTIONS returns the compile options as modified by any
       top-level (*XXX) option settings such as (*UTF) at the start of the
       pattern itself. PCRE2_INFO_EXTRAOPTIONS returns the extra options that
       were set in the compile context by calling the
       pcre2_set_compile_extra_options() function.

       For example, if the pattern /(*UTF)abc/ is compiled with the
       PCRE2_EXTENDED option, the result for PCRE2_INFO_ALLOPTIONS is
       PCRE2_EXTENDED and PCRE2_UTF. Option settings such as (?i) that can
       change within a pattern do not affect the result of
       PCRE2_INFO_ALLOPTIONS, even if they appear right at the start of the
       pattern. (This was different in some earlier releases.)

       A pattern compiled without PCRE2_ANCHORED is automatically anchored by
       PCRE2 if the first significant item in every top-level branch is one of
       the following:

           ^  unless PCRE2_MULTILINE is set
           \A always
           \G always
           .* sometimes - see below

       When .* is the first significant item, anchoring is possible only when
       all the following are true:

       o   .* is not in an atomic group

       o   .* is not in a capture group that is the subject of a backreference

       o   PCRE2_DOTALL is in force for .*

       o   Neither (*PRUNE) nor (*SKIP) appears in the pattern

       o   PCRE2_NO_DOTSTAR_ANCHOR is not set

       o   Dotstar anchoring has not been disabled with
           PCRE2_DOTSTAR_ANCHOR_OFF

       For patterns that are auto-anchored, the PCRE2_ANCHORED bit is set in
       the options returned for PCRE2_INFO_ALLOPTIONS.

           PCRE2_INFO_BACKREFMAX Return the number of the highest
                                 backreference in the pattern. The third
                                 argument should point to a uint32_t variable.
                                 Named capture groups acquire numbers as well
                                 as names, and these count towards the highest
                                 backreference. Backreferences such as \4 or
                                 \g{12} match the captured characters of the
                                 given group, but in addition, the check that
                                 a capture group is set in a conditional group
                                 such as (?(3)a|b) is also a backreference.
                                 Zero is returned if there are no
                                 backreferences.

           PCRE2_INFO_BSR The output is a uint32_t integer whose value
                          indicates what character sequences the \R escape
                          sequence matches. A value of PCRE2_BSR_UNICODE means
                          that \R matches any Unicode line ending sequence; a
                          value of PCRE2_BSR_ANYCRLF means that \R matches
                          only CR, LF, or CRLF.

           PCRE2_INFO_CAPTURECOUNT Return the highest capture group number in
                                   the pattern. In patterns where (?| is not
                                   used, this is also the total number of
                                   capture groups. The third argument should
                                   point to a uint32_t variable.

           PCRE2_INFO_DEPTHLIMIT If the pattern set a backtracking depth limit
                                 by including an item of the form
                                 (*LIMIT_DEPTH=nnnn) at the start, the value
                                 is returned. The third argument should point
                                 to a uint32_t integer. If no such value has
                                 been set, the call to pcre2_pattern_info()
                                 returns the error PCRE2_ERROR_UNSET. Note
                                 that this limit will only be used during
                                 matching if it is less than the limit set or
                                 defaulted by the caller of the match
                                 function.

           PCRE2_INFO_FIRSTBITMAP In the absence of a single first code unit
                                  for a non-anchored pattern, pcre2_compile()
                                  may construct a 256-bit table that defines a
                                  fixed set of values for the first code unit
                                  in any match. For example, a pattern that
                                  starts with [abc] results in a table with
                                  three bits set. When code unit values
                                  greater than 255 are supported, the flag bit
                                  for 255 means "any code unit of value 255 or
                                  above". If such a table was constructed, a
                                  pointer to it is returned. Otherwise NULL is
                                  returned. The third argument should point to
                                  a const uint8_t * variable.

           PCRE2_INFO_FIRSTCODETYPE
                                   Return information about the first code
                                   unit of any matched string, for a
                                   non-anchored pattern. The third argument
                                   should point to a uint32_t variable. If
                                   there is a fixed first value, for example,
                                   the letter "c" from a pattern such as
                                   (cat|cow|coyote), 1 is returned, and the
                                   value can be retrieved using
                                   PCRE2_INFO_FIRSTCODEUNIT. If there is no
                                   fixed first value, but it is known that a
                                   match can occur only at the start of the
                                   subject or following a newline in the
                                   subject, 2 is returned. Otherwise, and for
                                   anchored patterns, 0 is returned.

           PCRE2_INFO_FIRSTCODEUNIT
                                   Return the value of the first code unit of
                                   any matched string for a pattern where
                                   PCRE2_INFO_FIRSTCODETYPE returns 1;
                                   otherwise return 0. The third argument
                                   should point to a uint32_t variable. In the
                                   8-bit library, the value is always less
                                   than 256. In the 16-bit library the value
                                   can be up to 0xffff. In the 32-bit library
                                   in UTF-32 mode the value can be up to
                                   0x10ffff, and up to 0xffffffff when not
                                   using UTF-32 mode.

           PCRE2_INFO_FRAMESIZE Return the size (in bytes) of the data frames
                                that are used to remember backtracking
                                positions when the pattern is processed by
                                pcre2_match() without the use of JIT. The
                                third argument should point to a size_t
                                variable. The frame size depends on the number
                                of capturing parentheses in the pattern. Each
                                additional capture group adds two PCRE2_SIZE
                                variables.

           PCRE2_INFO_HASBACKSLASHC
                                   Return 1 if the pattern contains any
                                   instances of \C, otherwise 0. The third
                                   argument should point to a uint32_t
                                   variable.

           PCRE2_INFO_HASCRORLF Return 1 if the pattern contains any explicit
                                matches for CR or LF characters, otherwise 0.
                                The third argument should point to a uint32_t
                                variable. An explicit match is either a
                                literal CR or LF character, or \r or \n or one
                                of the equivalent hexadecimal or octal escape
                                sequences.

           PCRE2_INFO_HEAPLIMIT If the pattern set a heap memory limit by
                                including an item of the form
                                (*LIMIT_HEAP=nnnn) at the start, the value is
                                returned. The third argument should point to a
                                uint32_t integer. If no such value has been
                                set, the call to pcre2_pattern_info() returns
                                the error PCRE2_ERROR_UNSET. Note that this
                                limit will only be used during matching if it
                                is less than the limit set or defaulted by the
                                caller of the match function.

           PCRE2_INFO_JCHANGED Return 1 if the (?J) or (?-J) option setting is
                               used in the pattern, otherwise 0\. The third
                               argument should point to a uint32_t variable.
                               (?J) and (?-J) set and unset the local
                               PCRE2_DUPNAMES option, respectively.

           PCRE2_INFO_JITSIZE If the compiled pattern was successfully
                              processed by pcre2_jit_compile(), return the
                              size of the JIT compiled code, otherwise return
                              zero. The third argument should point to a
                              size_t variable.

           PCRE2_INFO_LASTCODETYPE Returns 1 if there is a rightmost literal
                                   code unit that must exist in any matched
                                   string, other than at its start. The third
                                   argument should point to a uint32_t
                                   variable. If there is no such value, 0 is
                                   returned. When 1 is returned, the code unit
                                   value itself can be retrieved using
                                   PCRE2_INFO_LASTCODEUNIT. For anchored
                                   patterns, a last literal value is recorded
                                   only if it follows something of variable
                                   length. For example, for the pattern
                                   /^a\d+z\d+/ the returned value is 1 (with
                                   "z" returned from PCRE2_INFO_LASTCODEUNIT),
                                   but for /^a\dz\d/ the returned value is 0.

           PCRE2_INFO_LASTCODEUNIT Return the value of the rightmost literal
                                   code unit that must exist in any matched
                                   string, other than at its start, for a
                                   pattern where PCRE2_INFO_LASTCODETYPE
                                   returns 1. Otherwise, return 0. The third
                                   argument should point to a uint32_t
                                   variable.

           PCRE2_INFO_MATCHEMPTY Return 1 if the pattern might match an empty
                                 string, otherwise 0. The third argument
                                 should point to a uint32_t variable. When a
                                 pattern contains recursive subroutine calls
                                 it is not always possible to determine
                                 whether or not it can match an empty string.
                                 PCRE2 takes a cautious approach and returns 1
                                 in such cases.

           PCRE2_INFO_MATCHLIMIT If the pattern set a match limit by including
                                 an item of the form (*LIMIT_MATCH=nnnn) at
                                 the start, the value is returned. The third
                                 argument should point to a uint32_t integer.
                                 If no such value has been set, the call to
                                 pcre2_pattern_info() returns the error
                                 PCRE2_ERROR_UNSET. Note that this limit will
                                 only be used during matching if it is less
                                 than the limit set or defaulted by the caller
                                 of the match function.

           PCRE2_INFO_MAXLOOKBEHIND
                                   A lookbehind assertion moves back a certain
                                   number of characters (not code units) when
                                   it starts to process each of its branches.
                                   This request returns the largest of these
                                   backward moves. The third argument should
                                   point to a uint32_t integer. The simple
                                   assertions \b and \B require a
                                   one-character lookbehind and cause
                                   PCRE2_INFO_MAXLOOKBEHIND to return 1 in the
                                   absence of anything longer. \A also
                                   registers a one-character lookbehind,
                                   though it does not actually inspect the
                                   previous character.

       Note that this information is useful for multi-segment matching only if
       the pattern contains no nested lookbehinds. For example, the pattern
       (?<=a(?<=ba)c) returns a maximum lookbehind of 2, but when it is
       processed, the first lookbehind moves back by two characters, matches
       one character, then the nested lookbehind also moves back by two
       characters. This puts the matching point three characters earlier than
       it was at the start. PCRE2_INFO_MAXLOOKBEHIND is really only useful as
       a debugging tool. See the pcre2partial documentation for a discussion
       of multi-segment matching.

           PCRE2_INFO_MINLENGTH If a minimum length for matching subject
                                strings was computed, its value is returned.
                                Otherwise the returned value is 0. This value
                                is not computed when PCRE2_NO_START_OPTIMIZE
                                is set. The value is a number of characters,
                                which in UTF mode may be different from the
                                number of code units. The third argument
                                should point to a uint32_t variable. The value
                                is a lower bound to the length of any matching
                                string. There may not be any strings of that
                                length that do actually match, but every
                                string that does match is at least that long.

           PCRE2_INFO_NAMECOUNT, PCRE2_INFO_NAMEENTRYSIZE,
           PCRE2_INFO_NAMETABLE
                                   PCRE2 supports the use of named as well as
                                   numbered capturing parentheses. The names
                                   are just an additional way of identifying
                                   the parentheses, which still acquire
                                   numbers. Several convenience functions such
                                   as pcre2_substring_get_byname() are
                                   provided for extracting captured substrings
                                   by name. It is also possible to extract the
                                   data directly, by first converting the name
                                   to a number in order to access the correct
                                   pointers in the output vector (described
                                   with pcre2_match() below). To do the
                                   conversion, you need to use the
                                   name-to-number map, which is described by
                                   these three values.

       The map consists of a number of fixed-size entries.
       PCRE2_INFO_NAMECOUNT gives the number of entries, and
       PCRE2_INFO_NAMEENTRYSIZE gives the size of each two bytes of each entry
       are the number of the capturing parenthesis, most significant byte
       first. In the 16-bit library, the pointer points to 16-bit code units,
       the first of which contains the parenthesis number. In the 32-bit
       library, the pointer points to 32-bit code units, the first of which
       contains the parenthesis number. The rest of the entry is the
       corresponding name, zero terminated.

       The names are in alphabetical order. If (?| is used to create multiple
       capture groups with the same number, as described in the section on
       duplicate group numbers in the pcre2pattern page, the groups may be
       given the same name, but there is only one entry in the table.
       Different names for groups of the same number are not permitted.

       Duplicate names for capture groups with different numbers are
       permitted, but only if PCRE2_DUPNAMES is set. They appear in the table
       in the order in which they were found in the pattern. In the absence of
       (?| this is the order of increasing number; when (?| is used this is
       not necessarily the case because later capture groups may have lower
       numbers.

       As a simple example of the name/number table, consider the following
       pattern after compilation by the 8-bit library (assume PCRE2_EXTENDED
       is set, so white space - including newlines - is ignored):

       (?<date> (?<year>(\d\d)?\d\d) -
       (?<month>\d\d) - (?<day>\d\d) )

       There are four named capture groups, so the table has four entries, and
       each entry in the table is eight bytes long. The table is as follows,
       with non-printing bytes shows in hexadecimal, and undefined bytes shown
       as ??:

       00 01 d  a  t  e  00 ??
       00 05 d  a  y  00 ?? ??
       00 04 m  o  n  t  h  00
       00 02 y  e  a  r  00 ??

       When writing code to extract data from named capture groups using the
       name-to-number map, remember that the length of the entries is likely
       to be different for each compiled pattern.

           PCRE2_INFO_NEWLINE The output is one of the following uint32_t
                              values:

           PCRE2_NEWLINE_CR      Carriage return (CR)
           PCRE2_NEWLINE_LF      Linefeed (LF)
           PCRE2_NEWLINE_CRLF    Carriage return, linefeed (CRLF)
           PCRE2_NEWLINE_ANY     Any Unicode line ending
           PCRE2_NEWLINE_ANYCRLF Any of CR, LF, or CRLF
           PCRE2_NEWLINE_NUL     The NUL character (binary zero)

       This identifies the character sequence that will be recognized as
       meaning "newline" while matching.

           PCRE2_INFO_SIZE Return the size of the compiled pattern in bytes
                           (for all three libraries). The third argument
                           should point to a size_t variable. This value
                           includes the size of the general data block that
                           precedes the code units of the compiled pattern
                           itself. The value that is used when pcre2_compile()
                           is getting memory in which to place the compiled
                           pattern may be slightly larger than the value
                           returned by this option, because there are cases
                           where the code that calculates the size has to
                           over-estimate. Processing a pattern with the JIT
                           compiler does not alter the value returned by this
                           option.


INFORMATION ABOUT A PATTERN'S CALLOUTS
       int pcre2_callout_enumerate(
             const pcre2_code *code,
             int (*callback)(pcre2_callout_enumerate_block *callout_block,
                             void *callout_data),
             void *callout_data);

       A script language that supports the use of string arguments in callouts
       might like to scan all the callouts in a pattern before running the
       match. This can be done by calling pcre2_callout_enumerate(). The first
       argument is a pointer to a compiled pattern, the second points to a
       callback function, and the third is arbitrary user data. The callback
       function is called for every callout in the pattern in the order in
       which they appear. Its first argument is a pointer to a callout
       enumeration block, and its second argument is the user_data value that
       was passed to pcre2_callout_enumerate(). The contents of the callout
       enumeration block are described in the pcre2callout documentation,
       which also gives further details about callouts.


SERIALIZATION AND PRECOMPILING
       It is possible to save compiled patterns on disc or elsewhere, and
       reload them later, subject to a number of restrictions. The host on
       which the patterns are reloaded must be running the same version of
       PCRE2, with the same code unit width, and must also have the same
       endianness, pointer width, and PCRE2_SIZE type. Before compiled
       patterns can be saved, they must be converted to a "serialized" form,
       which in the case of PCRE2 is really just a bytecode dump. The
       functions whose names begin with pcre2_serialize_ are used for
       converting to and from the serialized form. They are described in the
       pcre2serialize documentation. Note that PCRE2 serialization does not
       convert compiled patterns to an abstract format like Java or .NET
       serialization.


THE MATCH DATA BLOCK
       pcre2_match_data *pcre2_match_data_create(
             uint32_t oveccount, pcre2_general_context *gcontext);

       pcre2_match_data *pcre2_match_data_create_from_pattern(
             const pcre2_code *code, pcre2_general_context *gcontext);

       void pcre2_match_data_free(pcre2_match_data *match_data);

       Information about a successful or unsuccessful match is placed in a
       match data block, which is an opaque structure that is accessed by
       function calls. In particular, the match data block contains a vector
       of offsets into the subject string that define the matched parts of the
       subject. This is known as the ovector.

       Before calling pcre2_match(), pcre2_dfa_match(), or pcre2_jit_match()
       you must create a match data block by calling one of the creation
       functions above. For pcre2_match_data_create(), the first argument is
       the number of pairs of offsets in the ovector.

       When using pcre2_match(), one pair of offsets is required to identify
       the string that matched the whole pattern, with an additional pair for
       each captured substring. For example, a value of 4 creates enough space
       to record the matched portion of the subject plus three captured
       substrings.

       When using pcre2_dfa_match() there may be multiple matched substrings
       of different lengths at the same point in the subject. The ovector
       should be made large enough to hold as many as are expected.

       A minimum of at least 1 pair is imposed by pcre2_match_data_create(),
       so it is always possible to return the overall matched string in the
       case of pcre2_match() or the longest match in the case of
       pcre2_dfa_match(). The maximum number of pairs is 65535; if the first
       argument of pcre2_match_data_create() is greater than this, 65535 is
       used.

       The second argument of pcre2_match_data_create() is a pointer to a
       general context, which can specify custom memory management for
       obtaining the memory for the match data block. If you are not using
       custom memory management, pass NULL, which causes malloc() to be used.

       For pcre2_match_data_create_from_pattern(), the first argument is a
       pointer to a compiled pattern. The ovector is created to be exactly the
       right size to hold all the substrings a pattern might capture when
       matched using pcre2_match(). You should not use this call when matching
       with pcre2_dfa_match(). The second argument is again a pointer to a
       general context, but in this case if NULL is passed, the memory is
       obtained using the same allocator that was used for the compiled
       pattern (custom or default).

       A match data block can be used many times, with the same or different
       compiled patterns. You can extract information from a match data block
       after a match operation has finished, using functions that are
       described in the sections on matched strings and other match data
       below.

       When a call of pcre2_match() fails, valid data is available in the
       match block only when the error is PCRE2_ERROR_NOMATCH,
       PCRE2_ERROR_PARTIAL, or one of the error codes for an invalid UTF
       string. Exactly what is available depends on the error, and is detailed
       below.

       When one of the matching functions is called, pointers to the compiled
       pattern and the subject string are set in the match data block so that
       they can be referenced by the extraction functions after a successful
       match. After running a match, you must not free a compiled pattern or a
       subject string until after all operations on the match data block (for
       that match) have taken place, unless, in the case of the subject
       string, you have used the PCRE2_COPY_MATCHED_SUBJECT option, which is
       described in the section entitled "Option bits for pcre2_match()"
       below.

       When a match data block itself is no longer needed, it should be freed
       by calling pcre2_match_data_free(). If this function is called with a
       NULL argument, it returns immediately, without doing anything.


MEMORY USE FOR MATCH DATA BLOCKS
       PCRE2_SIZE pcre2_get_match_data_size(pcre2_match_data *match_data);

       PCRE2_SIZE pcre2_get_match_data_heapframes_size(
             pcre2_match_data *match_data);

       The size of a match data block depends on the size of the ovector that
       it contains. The function pcre2_get_match_data_size() returns the size,
       in bytes, of the block that is its argument.

       When pcre2_match() runs interpretively (that is, without using JIT), it
       makes use of a vector of data frames for remembering backtracking
       positions. The size of each individual frame depends on the number of
       capturing parentheses in the pattern and can be obtained by calling
       pcre2_pattern_info() with the PCRE2_INFO_FRAMESIZE option (see the
       section entitled "Information about a compiled pattern" above).

       Heap memory is used for the frames vector; if the initial memory block
       turns out to be too small during matching, it is automatically
       expanded. When pcre2_match() returns, the memory is not freed, but
       remains attached to the match data block, for use by any subsequent
       matches that use the same block. It is automatically freed when the
       match data block itself is freed.

       You can find the current size of the frames vector that a match data
       block owns by calling pcre2_get_match_data_heapframes_size(). For a
       newly created match data block the size will be zero. Some types of
       match may require a lot of frames and thus a large vector; applications
       that run in environments where memory is constrained can check this and
       free the match data block if the heap frames vector has become too big.


MATCHING A PATTERN: THE TRADITIONAL FUNCTION
       int pcre2_match(const pcre2_code *code, PCRE2_SPTR subject,
                       PCRE2_SIZE length, PCRE2_SIZE start_offset,
                       uint32_t options, pcre2_match_data *match_data,
                       pcre2_match_context *mcontext);

       The function pcre2_match() is called to match a subject string against
       a compiled pattern, which is passed in the code argument. You can call
       pcre2_match() with the same code argument as many times as you like, in
       order to find multiple matches in the subject string or to match
       different subject strings with the same pattern.

       This function is the main matching facility of the library, and it
       operates in a Perl-like manner. For specialist use there is also an
       alternative matching function, which is described below in the section
       about the pcre2_dfa_match() function.

       Here is an example of a simple call to pcre2_match():

       pcre2_match_data *md = pcre2_match_data_create(4, NULL);
       int rc = pcre2_match(
         re,             /* result of pcre2_compile() */
         "some string",  /* the subject string */
         11,             /* the length of the subject string */
         0,              /* start at offset 0 in the subject */
         0,              /* default options */
         md,             /* the match data block */
         NULL);          /* a match context; NULL means use defaults */

       If the subject string is zero-terminated, the length can be given as
       PCRE2_ZERO_TERMINATED. A match context must be provided if certain less
       common matching parameters are to be changed. For details, see the
       section on the match context above.

   The string to be matched by pcre2_match()
       The subject string is passed to pcre2_match() as a pointer in subject,
       a length in length, and a starting offset in start_offset. The length
       and offset are in code units, not characters. That is, they are in
       bytes for the 8-bit library, 16-bit code units for the 16-bit library,
       and 32-bit code units for the 32-bit library, whether or not UTF
       processing is enabled. As a special case, if subject is NULL and length
       is zero, the subject is assumed to be an empty string. If length is
       non-zero, an error occurs if subject is NULL.

       If start_offset is greater than the length of the subject,
       pcre2_match() returns PCRE2_ERROR_BADOFFSET. When the starting offset
       is zero, the search for a match starts at the beginning of the subject,
       and this is by far the most common case. In UTF-8 or UTF-16 mode, the
       starting offset must point to the start of a character, or to the end
       of the subject (in UTF-32 mode, one code unit equals one character, so
       all offsets are valid). Like the pattern string, the subject may
       contain binary zeros.

       A non-zero starting offset is useful when searching for another match
       in the same subject by calling pcre2_match() again after a previous
       success. Setting start_offset differs from passing over a shortened
       string and setting PCRE2_NOTBOL in the case of a pattern that begins
       with any kind of lookbehind. For example, consider the pattern

       \Biss\B

       which finds occurrences of "iss" in the middle of words. (\B matches
       only if the current position in the subject is not a word boundary.)
       When applied to the string "Mississippi" the first call to
       pcre2_match() finds the first occurrence. If pcre2_match() is called
       again with just the remainder of the subject, namely "issippi", it does
       not match, because \B is always false at the start of the subject,
       which is deemed to be a word boundary. However, if pcre2_match() is
       passed the entire string again, but with start_offset set to 4, it
       finds the second occurrence of iss because it is able to look behind
       the starting point to discover that it is preceded by a letter.

       Finding all the matches in a subject is tricky when the pattern can
       match an empty string. PCRE2 includes a helper API to assist with this;
       see the section entitled "Iterating over all matches" below for
       details.

       If a non-zero starting offset is passed when the pattern is anchored, a
       single attempt to match at the given offset is made. This can only
       succeed if the pattern does not require the match to be at the start of
       the subject. In other words, the anchoring must be the result of
       setting the PCRE2_ANCHORED option or the use of * with PCRE2_DOTALL,
       not by starting the pattern with ^ or \A.

   Option bits for pcre2_match()
       The unused bits of the options argument for pcre2_match() must be zero.
       The only bits that may be set are PCRE2_ANCHORED,
       PCRE2_COPY_MATCHED_SUBJECT, PCRE2_DISABLE_RECURSELOOP_CHECK,
       PCRE2_ENDANCHORED, PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY,
       PCRE2_NOTEMPTY_ATSTART, PCRE2_NO_JIT, PCRE2_NO_UTF_CHECK,
       PCRE2_PARTIAL_HARD, and PCRE2_PARTIAL_SOFT. Their action is described
       below.

       Setting PCRE2_ANCHORED or PCRE2_ENDANCHORED at match time is not
       supported by the just-in-time (JIT) compiler. If it is set, JIT
       matching is disabled and the interpretive code in pcre2_match() is run.
       PCRE2_DISABLE_RECURSELOOP_CHECK is ignored by JIT, but apart from
       PCRE2_NO_JIT (obviously), the remaining options are supported for JIT
       matching.

           PCRE2_ANCHORED          The PCRE2_ANCHORED option limits
                                   pcre2_match() to matching at the first
                                   matching position. If a pattern was
                                   compiled with PCRE2_ANCHORED, or turned out
                                   to be anchored by virtue of its contents,
                                   it cannot be made unanchored at matching
                                   time. Note that setting the option at match
                                   time disables JIT matching.
           PCRE2_COPY_MATCHED_SUBJECT
                                   By default, a pointer to the subject is
                                   remembered in the match data block so that,
                                   after a successful match, it can be
                                   referenced by the substring extraction
                                   functions. This means that the subject's
                                   memory must not be freed until all such
                                   operations are complete. For some
                                   applications where the lifetime of the
                                   subject string is not guaranteed, it may be
                                   necessary to make a copy of the subject
                                   string, but it is wasteful to do this
                                   unless the match is successful. After a
                                   successful match, if
                                   PCRE2_COPY_MATCHED_SUBJECT is set, the
                                   subject is copied and the new pointer is
                                   remembered in the match data block instead
                                   of the original subject pointer. The memory
                                   allocator that was used for the match block
                                   itself is used. The copy is automatically
                                   freed when pcre2_match_data_free() is
                                   called to free the match data block. It is
                                   also automatically freed if the match data
                                   block is re-used for another match
                                   operation.
           PCRE2_DISABLE_RECURSELOOP_CHECK
                                   This option is relevant only to
                                   pcre2_match() for interpretive matching. It
                                   is ignored when JIT is used, and is
                                   forbidden for pcre2_dfa_match().

       The use of recursion in patterns can lead to infinite loops. In the
       interpretive matcher these would be eventually caught by the match or
       heap limits, but this could take a long time and/or use a lot of memory
       if the limits are large. There is therefore a check at the start of
       each recursion. If the same group is still active from a previous call,
       and the current subject pointer is the same as it was at the start of
       that group, and the furthest inspected character of the subject has not
       changed, an error is generated.

       There are rare cases of matches that would complete, but nevertheless
       trigger this error. This option disables the check. It is provided
       mainly for testing when comparing JIT and interpretive behaviour.

       PCRE2_ENDANCHORED If the PCRE2_ENDANCHORED option is set, any string
                         that pcre2_match() matches must be right at the end
                         of the subject string. Note that setting the option
                         at match time disables JIT matching.

       PCRE2_NOTBOL      This option specifies that first character of the
                         subject string is not the beginning of a line, so the
                         circumflex metacharacter should not match before it.
                         Setting this without having set PCRE2_MULTILINE at
                         compile time causes circumflex never to match. This
                         option affects only the behaviour of the circumflex
                         metacharacter. It does not affect \A.

       PCRE2_NOTEOL      This option specifies that the end of the subject
                         string is not the end of a line, so the dollar
                         metacharacter should not match it nor (except in
                         multiline mode) a newline immediately before it.
                         Setting this without having set PCRE2_MULTILINE at
                         compile time causes dollar never to match. This
                         option affects only the behaviour of the dollar
                         metacharacter. It does not affect \Z or \z.

       PCRE2_NOTEMPTY    An empty string is not considered to be a valid match
                         if this option is set. If there are alternatives in
                         the pattern, they are tried. If all the alternatives
                         match the empty string, the entire match fails. For
                         example, if the pattern

       a?b?

       is applied to a string not beginning with "a" or "b", it matches an
       empty string at the start of the subject. With PCRE2_NOTEMPTY set, this
       match is not valid, so pcre2_match() searches further into the string
       for occurrences of "a" or "b".

       PCRE2_NOTEMPTY_ATSTART This is like PCRE2_NOTEMPTY, except that it
                              locks out an empty string match only at the
                              first matching position, that is, at the start
                              of the subject plus the starting offset. An
                              empty string match later in the subject is
                              permitted. If the pattern is anchored, such a
                              match can occur only if the pattern contains \K.

       PCRE2_NO_JIT           By default, if a pattern has been successfully
                              processed by pcre2_jit_compile(), JIT is
                              automatically used when pcre2_match() is called
                              with options that JIT supports. Setting
                              PCRE2_NO_JIT disables the use of JIT; it forces
                              matching to be done by the interpreter.

       PCRE2_NO_UTF_CHECK     When PCRE2_UTF is set at compile time, the
                              validity of the subject as a UTF string is
                              checked unless PCRE2_NO_UTF_CHECK is passed to
                              pcre2_match() or PCRE2_MATCH_INVALID_UTF was
                              passed to pcre2_compile(). The latter special
                              case is discussed in detail in the pcre2unicode
                              documentation.

       In the default case, if a non-zero starting offset is given, the check
       is applied only to that part of the subject that could be inspected
       during matching, and there is a check that the starting offset points
       to the first code unit of a character or to the end of the subject. If
       there are no lookbehind assertions in the pattern, the check starts at
       the starting offset. Otherwise, it starts at the length of the longest
       lookbehind before the starting offset, or at the start of the subject
       if there are not that many characters before the starting offset. Note
       that the sequences \b and \B are one-character lookbehinds.

       The check is carried out before any other processing takes place, and a
       negative error code is returned if the check fails. There are several
       UTF error codes for each code unit width, corresponding to different
       problems with the code unit sequence. There are discussions about the
       validity of UTF-8 strings, UTF-16 strings, and UTF-32 strings in the
       pcre2unicode documentation.

       If you know that your subject is valid, and you want to skip this check
       for performance reasons, you can set the PCRE2_NO_UTF_CHECK option when
       calling pcre2_match(). You might want to do this for the second and
       subsequent calls to pcre2_match() if you are making repeated calls to
       find multiple matches in the same subject string.

           Warning

           Unless PCRE2_MATCH_INVALID_UTF was set at compile time, when
           PCRE2_NO_UTF_CHECK is set at match time the effect of passing an
           invalid string as a subject, or an invalid value of start_offset,
           is undefined. Your program may crash or loop indefinitely or give
           wrong results.

       PCRE2_PARTIAL_HARD, PCRE2_PARTIAL_SOFT
                               These options turn on the partial matching
                               feature. A partial match occurs if the end of
                               the subject string is reached successfully, but
                               there are not enough subject characters to
                               complete the match. In addition, either at
                               least one character must have been inspected or
                               the pattern must contain a lookbehind, or the
                               pattern must be one that could match an empty
                               string.

       If this situation arises when PCRE2_PARTIAL_SOFT (but not
       PCRE2_PARTIAL_HARD) is set, matching continues by testing any remaining
       alternatives. Only if no complete match can be found is
       PCRE2_ERROR_PARTIAL returned instead of PCRE2_ERROR_NOMATCH. In other
       words, PCRE2_PARTIAL_SOFT specifies that the caller is prepared to
       handle a partial match, but only if no complete match can be found.

       If PCRE2_PARTIAL_HARD is set, it overrides PCRE2_PARTIAL_SOFT. In this
       case, if a partial match is found, pcre2_match() immediately returns
       PCRE2_ERROR_PARTIAL, without considering any other alternatives. In
       other words, when PCRE2_PARTIAL_HARD is set, a partial match is
       considered to be more important than an alternative complete match.

       There is a more detailed discussion of partial and multi-segment
       matching, with examples, in the pcre2partial documentation.


NEWLINE HANDLING WHEN MATCHING
       When PCRE2 is built, a default newline convention is set; this is
       usually the standard convention for the operating system. The default
       can be overridden in a compile context by calling pcre2_set_newline().
       It can also be overridden by starting a pattern string with, for
       example, (*CRLF), as described in the section on newline conventions in
       the pcre2pattern page. During matching, the newline choice affects the
       behaviour of the dot, circumflex, and dollar metacharacters. It may
       also alter the way the match starting position is advanced after a
       match failure for an unanchored pattern.

       When PCRE2_NEWLINE_CRLF, PCRE2_NEWLINE_ANYCRLF, or PCRE2_NEWLINE_ANY is
       set as the newline convention, and a match attempt for an unanchored
       pattern fails when the current starting position is at a CRLF sequence,
       and the pattern contains no explicit matches for CR or LF characters,
       the match position is advanced by two characters instead of one, in
       other words, to after the CRLF.

       The above rule is a compromise that makes the most common cases work as
       expected. For example, if the pattern is .+A (and the PCRE2_DOTALL
       option is not set), it does not match the string "\r\nA" because, after
       failing at the start, it skips both the CR and the LF before retrying.
       However, the pattern [\r\n]A does match that string, because it
       contains an explicit CR or LF reference, and so advances only by one
       character after the first failure.

       An explicit match for CR of LF is either a literal appearance of one of
       those characters in the pattern, or one of the \r or \n or equivalent
       octal or hexadecimal escape sequences. Implicit matches such as [^X] do
       not count, nor does \s, even though it includes CR and LF in the
       characters that it matches.

       Notwithstanding the above, anomalous effects may still occur when CRLF
       is a valid newline sequence and explicit \r or \n escapes appear in the
       pattern.


HOW pcre2_match() RETURNS A STRING AND CAPTURED SUBSTRINGS
       uint32_t pcre2_get_ovector_count(pcre2_match_data *match_data);

       PCRE2_SIZE *pcre2_get_ovector_pointer(pcre2_match_data *match_data);

       In general, a pattern matches a certain portion of the subject, and in
       addition, further substrings from the subject may be picked out by
       parenthesized parts of the pattern. Following the usage in Jeffrey
       Friedl's book, this is called "capturing" in what follows, and the
       phrase "capture group" (Perl terminology) is used for a fragment of a
       pattern that picks out a substring. PCRE2 supports several other kinds
       of parenthesized group that do not cause substrings to be captured. The
       pcre2_pattern_info() function can be used to find out how many capture
       groups there are in a compiled pattern.

       You can use auxiliary functions for accessing captured substrings by
       number or by name, as described in sections below.

       Alternatively, you can make direct use of the vector of PCRE2_SIZE
       values, called the ovector, which contains the offsets of captured
       strings. It is part of the match data block. The function
       pcre2_get_ovector_pointer() returns the address of the ovector, and
       pcre2_get_ovector_count() returns the number of pairs of values it
       contains.

       Within the ovector, the first in each pair of values is set to the
       offset of the first code unit of a substring, and the second is set to
       the offset of the first code unit after the end of a substring. These
       values are always code unit offsets, not character offsets. That is,
       they are byte offsets in the 8-bit library, 16-bit offsets in the
       16-bit library, and 32-bit offsets in the 32-bit library.

       After a partial match (error return PCRE2_ERROR_PARTIAL), only the
       first pair of offsets (that is, ovector[0] and ovector[1]) are set.
       They identify the part of the subject that was partially matched. See
       the pcre2partial documentation for details of partial matching.

       After a fully successful match, the first pair of offsets identifies
       the portion of the subject string that was matched by the entire
       pattern. The next pair is used for the first captured substring, and so
       on. The value returned by pcre2_match() is one more than the highest
       numbered pair that has been set. For example, if two substrings have
       been captured, the returned value is 3. If there are no captured
       substrings, the return value from a successful match is 1, indicating
       that just the first pair of offsets has been set.

       If a pattern uses the \K escape sequence within a positive lookahead
       assertion, the reported start of a successful match can be greater than
       the end of the match. For example, if the pattern (?=ab\K) is matched
       against "ab", the start and end offset values for the match are 2 and
       0.

       If a capture group is matched repeatedly within a single match
       operation, it is the last portion of the subject that it matched that
       is returned.

       If the ovector is too small to hold all the captured substring offsets,
       as much as possible is filled in, and the function returns a value of
       zero. If captured substrings are not of interest, pcre2_match() may be
       called with a match data block whose ovector is of minimum length (that
       is, one pair).

       It is possible for capture group number n+1 to match some part of the
       subject when group n has not been used at all. For example, if the
       string "abc" is matched against the pattern (a|(z[)](bc) the return
       from the function is 4, and groups 1 and 3 are matched, but 2 is not.
       When this happens, both values in the offset pairs corresponding to
       unused groups are set to PCRE2_UNSET.

       Offset values that correspond to unused groups at the end of the
       expression are also set to PCRE2_UNSET. For example, if the string
       "abc" is matched against the pattern (abc)(x(yz)?)? groups 2 and 3 are
       not matched. The return from the function is 2, because the highest
       used capture group number is 1. The offsets for the second and third
       capture groups (assuming the vector is large enough, of course) are set
       to PCRE2_UNSET.

       Elements in the ovector that do not correspond to capturing parentheses
       in the pattern are never changed. That is, if a pattern contains n
       capturing parentheses, no more than ovector[0] to ovector[2n+1] are set
       by pcre2_match(). The other elements retain whatever values they
       previously had. After a failed match attempt, the contents of the
       ovector are unchanged.


OTHER INFORMATION ABOUT A MATCH
       PCRE2_SPTR pcre2_get_mark(pcre2_match_data *match_data);

       PCRE2_SIZE pcre2_get_startchar(pcre2_match_data *match_data);

       As well as the offsets in the ovector, other information about a match
       is retained in the match data block and can be retrieved by the above
       functions in appropriate circumstances. If they are called at other
       times, the result is undefined.

       After a successful match, a partial match (PCRE2_ERROR_PARTIAL), or a
       failure to match (PCRE2_ERROR_NOMATCH), a mark name may be available.
       The function pcre2_get_mark() can be called to access this name, which
       can be specified in the pattern by any of the backtracking control
       verbs, not just ( *MARK). The same function applies to all the verbs.
       It returns a pointer to the zero-terminated name, which is within the
       compiled pattern. If no name is available, NULL is returned. The length
       of the name (excluding the terminating zero) is stored in the code unit
       that precedes the name. You should use this length instead of relying
       on the terminating zero if the name might contain a binary zero.

       After a successful match, the name that is returned is the last mark
       name encountered on the matching path through the pattern. Instances of
       backtracking verbs without names do not count. Thus, for example, if
       the matching path contains (*MARK:A)(*PRUNE), the name "A" is returned.
       After a "no match" or a partial match, the last encountered name is
       returned. For example, consider this pattern:

       ^(*MARK:A)((*MARK:B)a|b)c

       When it matches "bc", the returned name is A. The B mark is "seen" in
       the first branch of the group, but it is not on the matching path. On
       the other hand, when this pattern fails to match "bx", the returned
       name is B.

           Warning

           By default, certain start-of-match optimizations are used to give a
           fast "no match" result in some situations. For example, if the
           anchoring is removed from the pattern above, there is an initial
           check for the presence of "c" in the subject before running the
           matching engine. This check fails for "bx", causing a match failure
           without seeing any marks. You can disable the start-of-match
           optimizations by setting the PCRE2_NO_START_OPTIMIZE option for
           pcre2_compile() or by starting the pattern with (*NO_START_OPT).

       After a successful match, a partial match, or one of the invalid UTF
       errors (for example, PCRE2_ERROR_UTF8_ERR5), pcre2_get_startchar() can
       be called. After a successful or partial match it returns the code unit
       offset of the character at which the match started. For a non-partial
       match, this can be different to the value of ovector[0] if the pattern
       contains the \K escape sequence. After a partial match, however, this
       value is always the same as ovector[0] because \K does not affect the
       result of a partial match.

       After a UTF check failure, pcre2_get_startchar() can be used to obtain
       the code unit offset of the invalid UTF character. Details are given in
       the pcre2unicode page.


ERROR RETURNS FROM pcre2_match()
       If pcre2_match() fails, it returns a negative number. This can be
       converted to a text string by calling the pcre2_get_error_message()
       function (see "Obtaining a textual error message" below). Negative
       error codes are also returned by other functions, and are documented
       with them. The codes are given names in the header file. If UTF
       checking is in force and an invalid UTF subject string is detected, one
       of a number of UTF-specific negative error codes is returned. Details
       are given in the pcre2unicode page. The following are the other errors
       that may be returned by pcre2_match():

           PCRE2_ERROR_NOMATCH     The subject string did not match the
                                   pattern.
           PCRE2_ERROR_PARTIAL     The subject string did not match, but it
                                   did match partially. See the pcre2partial
                                   documentation for details of partial
                                   matching.
           PCRE2_ERROR_BADMAGIC    PCRE2 stores a 4-byte "magic number" at the
                                   start of the compiled code, to catch the
                                   case when it is passed a junk pointer. This
                                   is the error that is returned when the
                                   magic number is not present.
           PCRE2_ERROR_BADMODE     This error is given when a compiled pattern
                                   is passed to a function in a library of a
                                   different code unit width, for example, a
                                   pattern compiled by the 8-bit library is
                                   passed to a 16-bit or 32-bit library
                                   function.
           PCRE2_ERROR_BADOFFSET   The value of start_offset was greater than
                                   the length of the subject.
           PCRE2_ERROR_BADOPTION   An unrecognized bit was set in the options
                                   argument.
           PCRE2_ERROR_BADUTFOFFSET
                                   The UTF code unit sequence that was passed
                                   as a subject was checked and found to be
                                   valid (the PCRE2_NO_UTF_CHECK option was
                                   not set), but the value of start_offset did
                                   not point to the beginning of a UTF
                                   character or the end of the subject.
           PCRE2_ERROR_CALLOUT     This error is never generated by
                                   pcre2_match() itself. It is provided for
                                   use by callout functions that want to cause
                                   pcre2_match() or pcre2_callout_enumerate()
                                   to return a distinctive error code. See the
                                   pcre2callout documentation for details.
           PCRE2_ERROR_DEPTHLIMIT  The nested backtracking depth limit was
                                   reached.
           PCRE2_ERROR_HEAPLIMIT   The heap limit was reached.
           PCRE2_ERROR_INTERNAL    An unexpected internal error has occurred.
                                   This error could be caused by a bug in
                                   PCRE2 or by overwriting of the compiled
                                   pattern.
           PCRE2_ERROR_JIT_STACKLIMIT
                                   This error is returned when a pattern that
                                   was successfully studied using JIT is being
                                   matched, but the memory available for the
                                   just-in-time processing stack is not large
                                   enough. See the pcre2jit documentation for
                                   more details.
           PCRE2_ERROR_MATCHLIMIT  The backtracking match limit was reached.
           PCRE2_ERROR_NOMEMORY    Heap memory is used to remember
                                   backtracking points. This error is given
                                   when the memory allocation function
                                   (default or custom) fails. Note that a
                                   different error, PCRE2_ERROR_HEAPLIMIT, is
                                   given if the amount of memory needed
                                   exceeds the heap limit.
                                   PCRE2_ERROR_NOMEMORY is also returned if
                                   PCRE2_COPY_MATCHED_SUBJECT is set and
                                   memory allocation fails.
           PCRE2_ERROR_NULL        Either the code, subject, or match_data
                                   argument was passed as NULL.
           PCRE2_ERROR_RECURSELOOP This error is returned when pcre2_match()
                                   detects a recursion loop within the
                                   pattern. Specifically, it means that either
                                   the whole pattern or a capture group has
                                   been called recursively for the second time
                                   at the same position in the subject string.
                                   Some simple patterns that might do this are
                                   detected and faulted at compile time, but
                                   more complicated cases, in particular
                                   mutual recursions between two different
                                   groups, cannot be detected until matching
                                   is attempted.


OBTAINING A TEXTUAL ERROR MESSAGE
       int pcre2_get_error_message(int errorcode, PCRE2_UCHAR *buffer,
                                   PCRE2_SIZE bufflen);

       A text message for an error code from any PCRE2 function (compile,
       match, or auxiliary) can be obtained by calling
       pcre2_get_error_message(). The code is passed as the first argument,
       with the remaining two arguments specifying a code unit buffer and its
       length in code units, into which the text message is placed. The
       message is returned in code units of the appropriate width for the
       library that is being used.

       The returned message is terminated with a trailing zero, and the
       function returns the number of code units used, excluding the trailing
       zero. If the error number is unknown, the negative error code
       PCRE2_ERROR_BADDATA is returned. If the buffer is too small, the
       message is truncated (but still with a trailing zero), and the negative
       error code PCRE2_ERROR_NOMEMORY is returned. None of the messages is
       very long; a buffer size of 120 code units is ample.


ITERATING OVER ALL MATCHES
       int pcre2_next_match(pcre2_match_data *match_data,
                            PCRE2_SIZE *pstart_offset, uint32_t *poptions);

       A common task for applications is to implement "global" matching
       behaviour, for example, replacing all matches in the subject; splitting
       the subject on all matches; or simply counting the number of matches.
       The pcre2_next_match() function helps with this task by providing the
       appropriate parameters for the next match attempt (available since
       PCRE2 10.47).

       First, a match attempt should be made using one of the matching
       functions (pcre2_match(), pcre2_dfa_match(), or pcre2_jit_match()).
       Then, pcre2_next_match() can be called, providing the same match_data
       parameter.

       It returns 0 ("false") if there is no need to make a further match
       attempt, or 1 ("true") if another match should be attempted. Returning
       1 does not imply that there is another match, only that another match
       should be attempted (which may return PCRE2_ERROR_NOMATCH).

       The *pstart_offset and *poptions are set if the function returns 1. The
       *pstart_offset should be passed to the next match attempt directly, and
       the *poptions should be passed to the next match attempt by combining
       with the application's match options using OR.

       There is some code that demonstrates how to do this in the pcre2demo
       sample program. The general pattern is:

         uint32_t app_options = ...;
         uint32_t global_options = 0;
         PCRE2_SIZE start_offset = 0;
         while (1)
           {
           int rc = pcre2_match(re, subject, subject_len, start_offset,
                                app_options | global_options, match_data,
                                match_context);

           if (rc == PCRE2_ERROR_NOMATCH) break; /* no match, and no more attempts */
           if (rc < 0) { ... exit }

           ...handle the match

           if (!pcre2_next_match(match_data, &start_offset, &global_options))
             break; /* no more attempts */
           }

       The guarantees provided by pcre2_next_match() are that the start_offset
       will advance, so the loop will definitely terminate. The conditions
       which ensure this are that either:

       o   pcre2_next_match() returns 0 (false); or

       o   the returned *pstart_offset is strictly greater than the previous
           start_offset; or

       o   if the previous match was a successful match of the empty string,
           the returned *pstart_offset is equal to the previous ovector[1],
           and *poptions will be set to PCRE2_NOTEMPTY_ATSTART to prevent
           another empty match from being returned.

       A loop implemented as shown above will always terminate, unless there
       is a bug in PCRE2. As a measure of "defensive programming",
       applications are encouraged to add an assertion or check to break their
       loop if it does not make progress (and report the issue as a bug).

       If an application does not use the flag
       PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK, then each match is "well-behaved" and
       satisfies:

       start_offset <= ovector[0] <= ovector[1].

       In this case, the matches found by pcre2_match() with
       pcre2_next_match() will be sorted, non-overlapping (possibly touching),
       and with no duplicates.

       Otherwise, if PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK is used, then the
       guarantees are considerably weaker. We do not guarantee that the
       matches will always advance: only that the start_offset will. The
       matches found by pcre2_match() with pcre2_next_match() will be a finite
       sequence (as pcre2_next_match() ensures that start_offset advances, so
       the search will terminate). The matches can however be overlapping, can
       contain duplicates, and (in truly pathological examples) may not even
       be sorted by ovector[0]. Additionally, each match itself can end before
       it starts (ovector[1] < ovector[0]). We recommend that applications do
       not set PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK.


EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
       int pcre2_substring_length_bynumber(pcre2_match_data *match_data,
                                           uint32_t stringnumber,
                                           PCRE2_SIZE *sizeptr);

       int pcre2_substring_copy_bynumber(pcre2_match_data *match_data,
                                         uint32_t stringnumber,
                                         PCRE2_UCHAR *buffer,
                                         PCRE2_SIZE *sizeptr);

       int pcre2_substring_get_bynumber(pcre2_match_data *match_data,
                                        uint32_t stringnumber,
                                        PCRE2_UCHAR **stringptr,
                                        PCRE2_SIZE *sizeptr);

       void pcre2_substring_free(PCRE2_UCHAR *string);

       Captured substrings can be accessed directly by using the ovector as
       described above. For convenience, auxiliary functions are provided for
       extracting captured substrings as new, separate, zero-terminated
       strings. A substring that contains a binary zero is correctly extracted
       and has a further zero added on the end, but the result is not, of
       course, a C string.

       The functions in this section identify substrings by number. The number
       zero refers to the entire matched substring, with higher numbers
       referring to substrings captured by parenthesized groups. After a
       partial match, only substring zero is available. An attempt to extract
       any other substring gives the error PCRE2_ERROR_PARTIAL. The next
       section describes similar functions for extracting captured substrings
       by name.

       If a pattern uses the \K escape sequence within a positive lookahead
       assertion, the reported start of a successful match can be greater than
       the end of the match. For example, if the pattern (?=ab\K) is matched
       against "ab", the start and end offset values for the match are 2 and
       0. In this situation, calling these functions with a zero substring
       number extracts a zero-length empty string.

       You can find the length in code units of a captured substring without
       extracting it by calling pcre2_substring_length_bynumber(). The first
       argument is a pointer to the match data block, the second is the group
       number, and the third is a pointer to a variable into which the length
       is placed. If you just want to know whether or not the substring has
       been captured, you can pass the third argument as NULL.

       The pcre2_substring_copy_bynumber() function copies a captured
       substring into a supplied buffer, whereas
       pcre2_substring_get_bynumber() copies it into new memory, obtained
       using the same memory allocation function that was used for the match
       data block. The first two arguments of these functions are a pointer to
       the match data block and a capture group number.

       The final arguments of pcre2_substring_copy_bynumber() are a pointer to
       the buffer and a pointer to a variable that contains its length in code
       units. This is updated to contain the actual number of code units used
       for the extracted substring, excluding the terminating zero.

       For pcre2_substring_get_bynumber() the third and fourth arguments point
       to variables that are updated with a pointer to the new memory and the
       number of code units that comprise the substring, again excluding the
       terminating zero. When the substring is no longer needed, the memory
       should be freed by calling pcre2_substring_free().

       The return value from all these functions is zero for success, or a
       negative error code. If the pattern match failed, the match failure
       code is returned. If a substring number greater than zero is used after
       a partial match, PCRE2_ERROR_PARTIAL is returned. Other possible error
       codes are:

       PCRE2_ERROR_NOMEMORY

       The buffer was too small for pcre2_substring_copy_bynumber(), or the
       attempt to get memory failed for pcre2_substring_get_bynumber().

       PCRE2_ERROR_NOSUBSTRING

       There is no substring with that number in the pattern, that is, the
       number is greater than the number of capturing parentheses.

       PCRE2_ERROR_UNAVAILABLE

       The substring number, though not greater than the number of captures in
       the pattern, is greater than the number of slots in the ovector, so the
       substring could not be captured.

       PCRE2_ERROR_UNSET

       The substring did not participate in the match. For example, if the
       pattern is (abc)|(def) and the subject is "def", and the ovector
       contains at least two capturing slots, substring number 1 is unset.


EXTRACTING A LIST OF ALL CAPTURED SUBSTRINGS
       int pcre2_substring_list_get(pcre2_match_data *match_data,
                                    PCRE2_UCHAR ***listptr,
                                    PCRE2_SIZE **lengthsptr);

       void pcre2_substring_list_free(PCRE2_UCHAR **list);

       The pcre2_substring_list_get() function extracts all available
       substrings and builds a list of pointers to them. It also (optionally)
       builds a second list that contains their lengths (in code units),
       excluding a terminating zero that is added to each of them. All this is
       done in a single block of memory that is obtained using the same memory
       allocation function that was used to get the match data block.

       This function must be called only after a successful match. If called
       after a partial match, the error code PCRE2_ERROR_PARTIAL is returned.

       The address of the memory block is returned via listptr, which is also
       the start of the list of string pointers. The end of the list is marked
       by a NULL pointer. The address of the list of lengths is returned via
       lengthsptr. If your strings do not contain binary zeros and you do not
       therefore need the lengths, you may supply NULL as the lengthsptr
       argument to disable the creation of a list of lengths. The yield of the
       function is zero if all went well, or PCRE2_ERROR_NOMEMORY if the
       memory block could not be obtained. When the list is no longer needed,
       it should be freed by calling pcre2_substring_list_free().

       If this function encounters a substring that is unset, which can happen
       when capture group number n+1 matches some part of the subject, but
       group n has not been used at all, it returns an empty string. This can
       be distinguished from a genuine zero-length substring by inspecting the
       appropriate offset in the ovector, which contain PCRE2_UNSET for unset
       substrings, or by calling pcre2_substring_length_bynumber().


EXTRACTING CAPTURED SUBSTRINGS BY NAME
       int pcre2_substring_number_from_name(const pcre2_code *code,
                                            PCRE2_SPTR stringname);

       int pcre2_substring_length_byname(pcre2_match_data *match_data,
                                         PCRE2_SPTR stringname,
                                         PCRE2_SIZE *sizeptr);

       int pcre2_substring_copy_byname(pcre2_match_data *match_data,
                                       PCRE2_SPTR stringname,
                                       PCRE2_UCHAR *buffer,
                                       PCRE2_SIZE *sizeptr);

       int pcre2_substring_get_byname(pcre2_match_data *match_data,
                                      PCRE2_SPTR stringname,
                                      PCRE2_UCHAR **stringptr,
                                      PCRE2_SIZE *sizeptr);

       void pcre2_substring_free(PCRE2_UCHAR *string);

       To extract a substring by name, you first have to find associated
       number. For example, for this pattern:

       (a+)b(?<xxx>\d+)...

       the number of the capture group called "xxx" is 2. If the name is known
       to be unique (PCRE2_DUPNAMES was not set), you can find the number from
       the name by calling pcre2_substring_number_from_name(). The first
       argument is the compiled pattern, and the second is the name. The yield
       of the function is the group number, PCRE2_ERROR_NOSUBSTRING if there
       is no group with that name, or PCRE2_ERROR_NOUNIQUESUBSTRING if there
       is more than one group with that name. Given the number, you can
       extract the substring directly from the ovector, or use one of the
       "bynumber" functions described above.

       For convenience, there are also "byname" functions that correspond to
       the "bynumber" functions, the only difference being that the second
       argument is a name instead of a number. If PCRE2_DUPNAMES is set and
       there are duplicate names, these functions scan all the groups with the
       given name, and return the captured substring from the first named
       group that is set.

       If there are no groups with the given name, PCRE2_ERROR_NOSUBSTRING is
       returned. If all groups with the name have numbers that are greater
       than the number of slots in the ovector, PCRE2_ERROR_UNAVAILABLE is
       returned. If there is at least one group with a slot in the ovector,
       but no group is found to be set, PCRE2_ERROR_UNSET is returned.

           Warning

           If the pattern uses the (?| feature to set up multiple capture
           groups with the same number, as described in the section on
           duplicate group numbers in the pcre2pattern page, you cannot use
           names to distinguish the different capture groups, because names
           are not included in the compiled code. The matching process uses
           only numbers. For this reason, the use of different names for
           groups with the same number causes an error at compile time.


CREATING A NEW STRING WITH SUBSTITUTIONS
       int pcre2_substitute(const pcre2_code *code, PCRE2_SPTR subject,
                            PCRE2_SIZE length, PCRE2_SIZE start_offset,
                            uint32_t options, pcre2_match_data *match_data,
                            pcre2_match_context *mcontext,
                            PCRE2_SPTR replacement, PCRE2_SIZE rlength,
                            PCRE2_UCHAR *buffer, PCRE2_SIZE *blength);

       This function optionally calls pcre2_match() and then makes a copy of
       the subject string in buffer, replacing parts that were matched with
       the replacement string, whose length is supplied in rlength, which can
       be given as PCRE2_ZERO_TERMINATED for a zero-terminated string. As a
       special case, if replacement is NULL and rlength is zero, the
       replacement is assumed to be an empty string. If rlength is non-zero,
       an error occurs if replacement is NULL.

       There is an option (see PCRE2_SUBSTITUTE_REPLACEMENT_ONLY below) to
       return just the replacement string(s). The default action is to perform
       just one replacement if the pattern matches, but there is an option
       that requests multiple replacements (see PCRE2_SUBSTITUTE_GLOBAL
       below).

       If successful, pcre2_substitute() returns the number of substitutions
       that were carried out. This may be zero if no match was found, and is
       never greater than one unless PCRE2_SUBSTITUTE_GLOBAL is set. A
       negative value is returned if an error is detected.

       Matches in which a \K item in a lookahead in the pattern causes the
       match to end before it starts are not supported, and give rise to an
       error return. For global replacements, matches in which \K in a
       lookbehind causes the match to start earlier than the point that was
       reached in the previous iteration are also not supported. (These cases
       are only possible if the pattern was compiled with the
       backwards-compatibility option PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK.)

       The first seven arguments of pcre2_substitute() are the same as for
       pcre2_match(), except that the partial matching options are not
       permitted, and match_data may be passed as NULL, in which case a match
       data block is obtained and freed within this function, using memory
       management functions from the match context, if provided, or else those
       that were used to allocate memory for the compiled code.

       If match_data is not NULL and PCRE2_SUBSTITUTE_MATCHED is not set, the
       provided block is used for all calls to pcre2_match(), and its contents
       afterwards are the result of the final call made internally by
       pcre2_substitute() to the matching function. For global changes, this
       will always be a no-match error. The contents of the ovector within the
       match data block may or may not have been changed.

       As well as the usual options for pcre2_match(), a number of additional
       options can be set in the options argument of pcre2_substitute(). One
       such option is PCRE2_SUBSTITUTE_MATCHED. When this is set, an external
       match_data block must be provided, and it must have already been used
       for an external call to pcre2_match() (or pcre2_jit_match()) with the
       same pattern, subject pointer, effective subject length, start offset,
       and match option arguments (substitute-specific options can be added to
       the options argument). If any of these parameters is changed,
       pcre2_substitute() returns an error. The data in the match_data block
       (return code, offset vector) is used for the first substitution instead
       of calling pcre2_match() from within pcre2_substitute(). This allows an
       application to check for a match before choosing to substitute, without
       having to repeat the match.

       If the contents of the subject buffer are mutated in between
       pcre2_match() and a call to pcre2_substitute() with
       PCRE2_SUBSTITUTE_MATCHED, the behaviour is unsafe; in particular, in
       this case, PCRE2 is unable to ensure that the offsets in the ovector
       point to the start of characters (with UTF-encoded input).

       The contents of the externally supplied match data block are not
       changed when PCRE2_SUBSTITUTE_MATCHED is set, and so the match block is
       permitted for use in another call using PCRE2_SUBSTITUTE_MATCHED. If
       PCRE2_SUBSTITUTE_GLOBAL is also set, pcre2_match() is called after the
       first substitution to check for furthe matches, but this is done using
       an internally obtained match data block, thus always leaving the
       external block unchanged.

       The code argument is not used for matching before the first
       substitution when PCRE2_SUBSTITUTE_MATCHED is set, but it must be
       provided, even when PCRE2_SUBSTITUTE_GLOBAL is not set, because it
       contains information such as the UTF setting and the number of
       capturing parentheses in the pattern.

       When using PCRE2_SUBSTITUTE_MATCHED, you should not modify the subject
       string in between the prior call to pcre2_match() and
       pcre2_substitute(), as the substitution assumes that the passed-in
       ovector is compatible with the subject string. Although PCRE2 does
       verify that the subject is a pointer to the same buffer, it cannot in
       general verify whether the contents of the buffer have changed. For
       example, if the subject buffer is mutated from one valid UTF-8 string
       to another valid string, of the same length in code units, the ovector
       offsets are no longer guaranteed to point to the start of a character.
       Beware that with PCRE2_SUBSTITUTE_MATCHED in UTF mode, the subject
       string is not re-scanned for UTF validity when pcre2_substitute() first
       uses it.

       The default action of pcre2_substitute() is to return a copy of the
       subject string with matched substrings replaced. However, if
       PCRE2_SUBSTITUTE_REPLACEMENT_ONLY is set, only the replacement
       substrings are returned. In the global case, multiple replacements are
       concatenated in the output buffer. Substitution callouts (see below)
       can be used to separate them if necessary.

       Partial matching is supported, with limitations: if matching succeeds
       but with a partial match, then pcre2_substitute() returns
       PCRE2_ERROR_PARTIAL. When partial-matching (either of
       PCRE2_PARTIAL_HARD or PCRE2_PARTIAL_SOFT is passed), then
       PCRE2_SUBSTITUTE_REPLACEMENT_ONLY must also be set, or else
       PCRE2_ERROR_BADOPTION is returned. Similarly, certain replacement items
       ($' and $_) cause PCRE2_ERROR_PARTIALSUBS to be returned when
       partial-matching, even if a complete match is found.

       The blength argument of pcre2_substitute() must point to a variable
       that contains the length, in code units, of the output buffer. If the
       function is successful, the value is updated to contain the length in
       code units of the new string, excluding the trailing zero that is
       automatically added.

       If the function is not successful, the value set via blength depends on
       the type of error. For syntax errors in the replacement string, the
       value is the offset in the replacement string where the error was
       detected. For other errors, the value is PCRE2_UNSET by default. This
       includes the case of the output buffer being too small, unless
       PCRE2_SUBSTITUTE_OVERFLOW_LENGTH is set.

       PCRE2_SUBSTITUTE_OVERFLOW_LENGTH changes what happens when the output
       buffer is too small. The default action is to return
       PCRE2_ERROR_NOMEMORY immediately. If this option is set, however,
       pcre2_substitute() continues to go through the motions of matching and
       substituting (without, of course, writing anything) in order to compute
       the size of buffer that is needed, which will include the extra space
       for the terminating NUL. This value is passed back via the blength
       variable, with the result of the function still being
       PCRE2_ERROR_NOMEMORY.

       Passing a buffer size of zero (with buffer optionally NULL) is a
       permitted way of finding out how much memory is needed for given
       substitution. However, this does mean that the entire operation is
       carried out twice. Depending on the application, it may be more
       efficient to allocate a large buffer and free the excess afterwards,
       instead of using PCRE2_SUBSTITUTE_OVERFLOW_LENGTH.

       The replacement string, which is interpreted as a UTF string in UTF
       mode, is checked for UTF validity unless PCRE2_NO_UTF_CHECK is set. An
       invalid UTF replacement string causes an immediate return with the
       relevant UTF error code.

       If PCRE2_SUBSTITUTE_LITERAL is set, the replacement string is not
       interpreted in any way. By default, however, a dollar character is an
       escape character that can specify the insertion of characters from
       capture groups and names from (*MARK) or other control verbs in the
       pattern. Dollar is the only escape character (backslash is treated as
       literal). The following forms are recognized:

       $$                  insert a dollar character
       $n or ${n}          insert the contents of group _n_
       $0 or $&            insert the entire matched substring
       $`                  insert the substring that precedes the match
       $'                  insert the substring that follows the match
       $_                  insert the entire input string
       $+                  insert the highest-numbered capture group which matched
       $*MARK or ${*MARK}  insert a control verb name

       Either a group number or a group name can be given for n, for example
       $2 or $NAME. Curly brackets are required only if the following
       character would be interpreted as part of the number or name. The
       number may be zero to include the entire matched string. For example,
       if the pattern a(b)c is matched with "=abc=" and the replacement string
       $1$0$1, the result is "=babcb=".

       The JavaScript form $<name>, where the angle brackets are part of the
       syntax, is also recognized for group names, but not for group numbers
       or *MARK.

       The $*MARK inserts the name from the last encountered backtracking
       control verb on the matching path that has a name. (*MARK) must always
       include a name, but the other verbs need not. For example, in the case
       of (*MARK:A)(*PRUNE) the name inserted is "A", but for
       (*MARK:A)(*PRUNE:B) the relevant name is "B". This facility can be used
       to perform simple simultaneous substitutions, as this pcre2test example
       shows:

       /(*MARK:pear)apple|(*MARK:orange)lemon/g,replace=${*MARK}
           apple lemon
        2: pear orange

       PCRE2_SUBSTITUTE_GLOBAL causes the function to iterate over the subject
       string, replacing every matching substring. If this option is not set,
       only the first matching substring is replaced. The search for matches
       takes place in the original subject string (that is, previous
       replacements do not affect it). Iteration is implemented by advancing
       the start_offset value for each search, which is always passed the
       entire subject string. If an offset limit is set in the match context,
       searching stops when that limit is reached.

       Because global substitutions apply the pattern repeatedly to the
       subject string, and always iterate over non-overlapping matches, the
       substitutions done by pcre2_substitute() do not match and substitute
       text inside the replacement strings themselves (no recursive/iterative
       substitution). However, applications can easily implement other
       alternative replacement strategies, such as iteratively replacing, then
       matching and replacing on the result. The replacement loop inside
       pcre2_substitute() is simple and can be emulated in client code by
       allocating a buffer, searching for matches in a loop, and calling
       pcre2_substitute() with PCRE2_SUBSTITUTE_REPLACEMENT_ONLY an
       PCRE2_SUBSTITUTE_MATCHED, and without PCRE2_SUBSTITUTE_GLOBAL.

       You can restrict the effect of a global substitution to a portion of
       the subject string by setting either or both of start_offset and an
       offset limit. Here is a pcre2test example:

       /B/g,replace=!,use_offset_limit
       ABC ABC ABC ABC\=offset=3,offset_limit=12
        2: ABC A!C A!C ABC

       When continuing with global substitutions after matching a substring
       with zero length, an attempt to find a non-empty match at the same
       offset is performed. If this is not successful, the offset is advanced
       by one character except when CRLF is a valid newline sequence and the
       next two characters are CR, LF. In this case, the offset is advanced by
       two characters.

       PCRE2_SUBSTITUTE_UNKNOWN_UNSET causes references to capture groups that
       do not appear in the pattern to be treated as unset groups. This option
       should be used with care, because it means that a typo in a group name
       or number no longer causes the PCRE2_ERROR_NOSUBSTRING error.

       PCRE2_SUBSTITUTE_UNSET_EMPTY causes unset capture groups (including
       unknown groups when PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set) to be
       treated as empty strings when inserted as described above. If this
       option is not set, an attempt to insert an unset group causes the
       PCRE2_ERROR_UNSET error. This option does not influence the extended
       substitution syntax described below.

       PCRE2_SUBSTITUTE_EXTENDED causes extra processing to be applied to the
       replacement string. Without this option, only the dollar character is
       special, and only the group insertion forms listed above are valid.
       When PCRE2_SUBSTITUTE_EXTENDED is set, several things change:

       Firstly, backslash in a replacement string is interpreted as an escape
       character. The usual forms such as \x{ddd} can be used to specify
       particular character codes, and backslash followed by any
       non-alphanumeric character quotes that character. Extended quoting can
       be coded using \Q...\E, exactly as in pattern strings. The escapes \b
       and \v are interpreted as the characters backspace and vertical tab,
       respectively.

       The interpretation of backslash followed by one or more digits is the
       same as in a pattern, which in Perl has some ambiguities. Details are
       given in the pcre2pattern page.

       The Python form \g<n>, where the angle brackets are part of the syntax
       and n is either a group name or number, is recognized as an alternative
       way of inserting the contents of a group, for example \g<3>.

       There are also four escape sequences for forcing the case of inserted
       letters. Case forcing applies to all inserted characters, including
       those from capture groups and letters within \Q...\E quoted sequences.
       The insertion mechanism has three states: no case forcing, force upper
       case, and force lower case. The escape sequences change the current
       state: \U and \L change to upper or lower case forcing, respectively,
       and \E (when not terminating a \Q quoted sequence) reverts to no case
       forcing. The sequences \u and \l force the next character (if it is a
       letter) to upper or lower case, respectively, and then the state
       automatically reverts to no case forcing.

       However, if \u is immediately followed by \L or \l is immediately
       followed by \U, the next character's case is forced by the first escape
       sequence, and subsequent characters by the second. This provides a
       "title casing" facility that can be applied to group captures. For
       example, if group 1 has captured "heLLo", the replacement string \u\L$1
       becomes "Hello".

       If either PCRE2_UTF or PCRE2_UCP was set when the pattern was compiled,
       Unicode properties are used for case forcing characters whose code
       points are greater than 127. However, only simple case folding, as
       determined by the Unicode file CaseFolding.txt is supported. PCRE2 does
       not support language-specific special casing rules such as using
       different lower case Greek sigmas in the middle and ends of words (as
       defined in the Unicode file SpecialCasing.txt).

       Note that case forcing sequences such as \U...\E do not nest. For
       example, the result of processing \Uaa\LBB\Ecc\E is "AAbbcc"; the final
       \E has no effect. Note also that the PCRE2_ALT_BSUX and
       PCRE2_EXTRA_ALT_BSUX options do not apply to replacement strings.

       The final effect of setting PCRE2_SUBSTITUTE_EXTENDED is to add more
       flexibility to capture group substitution. The syntax is similar to
       that used by Bash:

       ${n:-string}
       ${n:+string1:string2}

       As in the simple case, n may be a group number or a name. The first
       form specifies a default value. If group n is set, its value is
       inserted; if not, the string is expanded and the result inserted. The
       second form specifies strings that are expanded and inserted when group
       n is set or unset, respectively. The first form is just a convenient
       shorthand for

       ${n:+${n}:string}

       Backslash can be used to escape colons and closing curly brackets in
       the replacement strings. A change of the case forcing state within a
       replacement string remains in force afterwards, as shown in this
       pcre2test example:

       /(some)?(body)/substitute_extended,replace=${1:+\U:\L}HeLLo
           body
        1: hello
           somebody
        1: HELLO

       The PCRE2_SUBSTITUTE_UNSET_EMPTY option does not affect these extended
       substitutions. However, PCRE2_SUBSTITUTE_UNKNOWN_UNSET does cause
       unknown groups in the extended syntax forms to be treated as unset.

       If PCRE2_SUBSTITUTE_LITERAL is set, PCRE2_SUBSTITUTE_UNKNOWN_UNSET,
       PCRE2_SUBSTITUTE_UNSET_EMPTY, and PCRE2_SUBSTITUTE_EXTENDED are
       irrelevant and are ignored.

   Substitution errors
       In the event of an error, pcre2_substitute() returns a negative error
       code. Except for PCRE2_ERROR_NOMATCH (which is never returned), errors
       from pcre2_match() are passed straight back.

           PCRE2_ERROR_NOSUBSTRING is returned for a non-existent substring
                                   insertion, unless
                                   PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set.
           PCRE2_ERROR_UNSET       is returned for an unset substring
                                   insertion (including an unknown substring
                                   when PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set)
                                   when the simple (non-extended) syntax is
                                   used and PCRE2_SUBSTITUTE_UNSET_EMPTY is
                                   not set.
           PCRE2_ERROR_NOMEMORY    is returned if the output buffer is not big
                                   enough. If the
                                   PCRE2_SUBSTITUTE_OVERFLOW_LENGTH option is
                                   set, the size of buffer that is needed is
                                   returned via blength. Note that this does
                                   not happen by default.
           PCRE2_ERROR_NULL        is returned if PCRE2_SUBSTITUTE_MATCHED is
                                   set but the match_data argument is NULL; or
                                   if the subject or replacement or buffer
                                   argument is NULL. However, a NULL
                                   replacement argument is allowed if the
                                   rlength argument is zero; and similarly for
                                   subject with zero length and buffer with
                                   zero blength.
           PCRE2_ERROR_BADREPLACEMENT
                                   is used for miscellaneous syntax errors in
                                   the replacement string, with more
                                   particular errors being
                                   PCRE2_ERROR_BADREPESCAPE (invalid escape
                                   sequence), PCRE2_ERROR_REPMISSINGBRACE
                                   (closing curly bracket not found),
                                   PCRE2_ERROR_BADSUBSTITUTION (syntax error
                                   in extended group substitution), and
                                   PCRE2_ERROR_BADSUBSPATTERN (the pattern
                                   match ended before it started or the match
                                   started earlier than the current position
                                   in the subject, which can happen if \K is
                                   used in a lookaround assertion).

       As for all PCRE2 errors, a text message that describes the error can be
       obtained by calling the pcre2_get_error_message() function (see
       "Obtaining a textual error message" above).

   Substitution callouts
       int pcre2_set_substitute_callout(
             pcre2_match_context *mcontext,
             int (*substitute_callout)(
                 pcre2_substitute_callout_block *callout_block,
                 void *substitute_callout_data),
             void *substitute_callout_data);

       The pcre2_set_substitute_callout() function can be used to specify a
       callout function for pcre2_substitute(). This information is passed in
       a match context. The callout function is called after each substitution
       has been processed, but it can cause the replacement not to happen.

       The callout function is not called for simulated substitutions that
       happen as a result of the PCRE2_SUBSTITUTE_OVERFLOW_LENGTH option. In
       this mode, when substitution processing exceeds the buffer space
       provided by the caller, processing continues by counting code units.
       The simulation is unable to populate the callout block, and so the
       simulation is pessimistic about the required buffer size. Whichever is
       larger of accepted or rejected substitution is reported as the required
       size. Therefore, the returned buffer length may be an overestimate
       (without a substitution callout, it is normally an exact measurement).

       The first argument of the callout function is a pointer to a substitute
       callout block structure, which contains the following fields, not
       necessarily in this order:

           uint32_t version;, uint32_t subscount;, PCRE2_SPTR input;,
           PCRE2_SPTR output;, PCRE2_SIZE *ovector;, uint32_t oveccount;,
           PCRE2_SIZE output_offsets[2];
                                   The version field contains the version
                                   number of the block format. The current
                                   version is 0. The version number will
                                   increase in future if more fields are
                                   added, but the intention is never to remove
                                   any of the existing fields.

       The subscount field is the number of the current match. It is 1 for the
       first callout, 2 for the second, and so on. The input and output
       pointers are copies of the values passed to pcre2_substitute().

       The ovector field points to the ovector, which contains the result of
       the most recent match. The oveccount field contains the number of pairs
       that are set in the ovector, and is always greater than zero.

       The output_offsets vector contains the offsets of the replacement in
       the output string. This has already been processed for dollar and (if
       requested) backslash substitutions as described above.

       The second argument of the callout function is the value passed as
       callout_data when the function was registered. The value returned by
       the callout function is interpreted as follows:

       If the value is zero, the replacement is accepted, and, if
       PCRE2_SUBSTITUTE_GLOBAL is set, processing continues with a search for
       the next match. If the value is not zero, the current replacement is
       not accepted. If the value is greater than zero, processing continues
       when PCRE2_SUBSTITUTE_GLOBAL is set. Otherwise (the value is less than
       zero or PCRE2_SUBSTITUTE_GLOBAL is not set), the rest of the input is
       copied to the output and the call to pcre2_substitute() exits,
       returning the number of matches so far.

   Substitution case callouts
       int pcre2_set_substitute_case_callout(
             pcre2_match_context *mcontext,
             PCRE2_SIZE (*substitute_case_callout)(
                 PCRE2_SPTR input, PCRE2_SIZE input_length,
                 PCRE2_UCHAR *output, PCRE2_SIZE output_capacity,
                 int case_operation, void *substitute_case_callout_data),
             void *substitute_case_callout_data);

       The pcre2_set_substitute_case_callout() function can be used to specify
       a callout function for pcre2_substitute() to use when performing case
       transformations. This does not affect any case insensitivity behaviour
       when performing a match, but only the user-visible transformations
       performed when processing a substitution such as:

         pcre2_substitute(..., "\\U$1", ...)

       The default case transformations applied by PCRE2 are reasonably
       complete, and, in UTF or UCP mode, perform the simple locale-invariant
       case transformations as specified by Unicode. This is suitable for the
       internal (invisible) case-equivalence procedures used during pattern
       matching, but an application may wish to use more sophisticated
       locale-aware processing for the user-visible substitution
       transformations.

       One example implementation of the callout_function using the ICU
       library would be:

           PCRE2_SIZE
           icu_case_callout(
             PCRE2_SPTR input, PCRE2_SIZE input_len,
             PCRE2_UCHAR *output, PCRE2_SIZE output_cap,
             int to_case, void *data_ptr)
           {
             UErrorCode err = U_ZERO_ERROR;
             int32_t r = to_case == PCRE2_SUBSTITUTE_CASE_LOWER
               ? u_strToLower(output, output_cap, input, input_len, NULL, &err)
               : to_case == PCRE2_SUBSTITUTE_CASE_UPPER
               ? u_strToUpper(output, output_cap, input, input_len, NULL, &err)
               : u_strToTitle(output, output_cap, input, input_len, &first_char_only,
                              NULL, &err);
             if (U_FAILURE(err)) return (~(PCRE2_SIZE)0);
             return r;
           }

       The first and second arguments of the case callout function are the
       Unicode string to transform.

       The third and fourth arguments are the output buffer and its capacity.

       The fifth is one of the constants PCRE2_SUBSTITUTE_CASE_LOWER,
       PCRE2_SUBSTITUTE_CASE_UPPER, or PCRE2_SUBSTITUTE_CASE_TITLE_FIRST.
       PCRE2_SUBSTITUTE_CASE_LOWER and PCRE2_SUBSTITUTE_CASE_UPPER are passed
       to the callout to indicate that the case of the entire callout input
       should be case-transformed. PCRE2_SUBSTITUTE_CASE_TITLE_FIRST is passed
       to indicate that only the first character or glyph should be
       transformed to Unicode titlecase and the rest to Unicode lowercase
       (note that titlecasing sometimes uses Unicode properties to titlecase
       each word in a string; but PCRE2 is requesting that only the single
       leading character is to be titlecased).

       The sixth argument is the callout_data supplied to
       pcre2_set_substitute_case_callout().

       The resulting string in the destination buffer may be larger or smaller
       than the input, if the casing rules merge or split characters. The
       return value is the length required for the output string. If a buffer
       of sufficient size was provided to the callout, then the result must be
       written to the buffer and the number of code units returned. If the
       result does not fit in the provided buffer, then the required capacity
       must be returned and PCRE2 will not make use of the output buffer.
       PCRE2 provides input and output buffers which overlap, so the callout
       must support this by suitable internal buffering.

       Alternatively, if the callout wishes to indicate an error, then it may
       return (~(PCRE2_SIZE)0). In this case pcre2_substitute() will
       immediately fail with error PCRE2_ERROR_REPLACECASE.

       When a case callout is combined with the
       PCRE2_SUBSTITUTE_OVERFLOW_LENGTH option, there are situations when
       pcre2_substitute() will return an underestimate of the required buffer
       size. If you call pcre2_substitute() once with
       PCRE2_SUBSTITUTE_OVERFLOW_LENGTH, and the input buffer is too small for
       the replacement string to be constructed, then instead of calling the
       case callout, pcre2_substitute() will make an estimate of the required
       buffer size. The second call should also pass
       PCRE2_SUBSTITUTE_OVERFLOW_LENGTH, because that second call is not
       guaranteed to succeed either, if the case callout requires more buffer
       space than expected. The caller must make repeated attempts in a loop.


DUPLICATE CAPTURE GROUP NAMES
       int pcre2_substring_nametable_scan(const pcre2_code *code,
                                          PCRE2_SPTR stringname,
                                          PCRE2_SPTR *firstptr,
                                          PCRE2_SPTR *lastptr);

       When a pattern is compiled with the PCRE2_DUPNAMES option, names for
       capture groups are not required to be unique. Duplicate names are
       always allowed for groups with the same number, created by using the
       (?| feature. Indeed, if such groups are named, they are required to use
       the same names.

       Normally, patterns that use duplicate names are such that in any one
       match, only one of each set of identically-named groups participates.
       An example is shown in the pcre2pattern documentation.

       When duplicates are present, pcre2_substring_copy_byname() and
       pcre2_substring_get_byname() return the first substring corresponding
       to the given name that is set. Only if none are set is
       PCRE2_ERROR_UNSET is returned. The pcre2_substring_number_from_name()
       function returns the error PCRE2_ERROR_NOUNIQUESUBSTRING when there are
       duplicate names.

       If you want to get full details of all captured substrings for a given
       name, you must use the pcre2_substring_nametable_scan() function. The
       first argument is the compiled pattern, and the second is the name. If
       the third and fourth arguments are NULL, the function returns a group
       number for a unique name, or PCRE2_ERROR_NOUNIQUESUBSTRING otherwise.

       When the third and fourth arguments are not NULL, they must be pointers
       to variables that are updated by the function. After it has run, they
       point to the first and last entries in the name-to-number table for the
       given name, and the function returns the length of each entry in code
       units. In both cases, PCRE2_ERROR_NOSUBSTRING is returned if there are
       no entries for the given name.

       The format of the name table is described above in the section entitled
       Information about a pattern. Given all the relevant entries for the
       name, you can extract each of their numbers, and hence the captured
       data.


FINDING ALL POSSIBLE MATCHES AT ONE POSITION
       The traditional matching function uses a similar algorithm to Perl,
       which stops when it finds the first match at a given point in the
       subject. If you want to find all possible matches, or the longest
       possible match at a given position, consider using the alternative
       matching function (see below) instead. If you cannot use the
       alternative function, you can kludge it up by making use of the callout
       facility, which is described in the pcre2callout documentation.

       What you have to do is to insert a callout right at the end of the
       pattern. When your callout function is called, extract and save the
       current matched substring. Then return 1, which forces pcre2_match() to
       backtrack and try other alternatives. Ultimately, when it runs out of
       matches, pcre2_match() will yield PCRE2_ERROR_NOMATCH.


MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
       int pcre2_dfa_match(const pcre2_code *code, PCRE2_SPTR subject,
                           PCRE2_SIZE length, PCRE2_SIZE start_offset,
                           uint32_t options, pcre2_match_data *match_data,
                           pcre2_match_context *mcontext, int *workspace,
                           PCRE2_SIZE wscount);

       The function pcre2_dfa_match() is called to match a subject string
       against a compiled pattern, using a matching algorithm that scans the
       subject string just once (not counting lookaround assertions), and does
       not backtrack (except when processing lookaround assertions). This has
       different characteristics to the normal algorithm, and is not
       compatible with Perl. Some of the features of PCRE2 patterns are not
       supported. Nevertheless, there are times when this kind of matching can
       be useful. For a discussion of the two matching algorithms, and a list
       of features that pcre2_dfa_match() does not support, see the
       pcre2matching documentation.

       The arguments for the pcre2_dfa_match() function are the same as for
       pcre2_match(), plus two extras. The ovector within the match data block
       is used in a different way, and this is described below. The other
       common arguments are used in the same way as for pcre2_match(), so
       their description is not repeated here.

       The two additional arguments provide workspace for the function. The
       workspace vector should contain at least 20 elements. It is used for
       keeping track of multiple paths through the pattern tree. More
       workspace is needed for patterns and subjects where there are a lot of
       potential matches.

       Here is an example of a simple call to pcre2_dfa_match():

       int wspace[20];
       pcre2_match_data *md = pcre2_match_data_create(4, NULL);
       int rc = pcre2_dfa_match(
         re,             /* result of pcre2_compile() */
         "some string",  /* the subject string */
         11,             /* the length of the subject string */
         0,              /* start at offset 0 in the subject */
         0,              /* default options */
         md,             /* the match data block */
         NULL,           /* a match context; NULL means use defaults */
         wspace,         /* working space vector */
         20);            /* number of elements (NOT size in bytes) */

   Option bits for pcre2_dfa_match()
       The unused bits of the options argument for pcre2_dfa_match() must be
       zero. The only bits that may be set are PCRE2_ANCHORED,
       PCRE2_COPY_MATCHED_SUBJECT, PCRE2_ENDANCHORED, PCRE2_NOTBOL,
       PCRE2_NOTEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART,
       PCRE2_NO_UTF_CHECK, PCRE2_PARTIAL_HARD, PCRE2_PARTIAL_SOFT,
       PCRE2_DFA_SHORTEST, and PCRE2_DFA_RESTART. All but the last four of
       these are exactly the same as for pcre2_match(), so their description
       is not repeated here.

       PCRE2_PARTIAL_HARD
       PCRE2_PARTIAL_SOFT

       These have the same general effect as they do for pcre2_match(), but
       the details are slightly different. When PCRE2_PARTIAL_HARD is set for
       pcre2_dfa_match(), it returns PCRE2_ERROR_PARTIAL if the end of the
       subject is reached and there is still at least one matching possibility
       that requires additional characters. This happens even if some complete
       matches have already been found. When PCRE2_PARTIAL_SOFT is set, the
       return code PCRE2_ERROR_NOMATCH is converted into PCRE2_ERROR_PARTIAL
       if the end of the subject is reached, there have been no complete
       matches, but there is still at least one matching possibility. The
       portion of the string that was inspected when the longest partial match
       was found is set as the first matching string in both cases. There is a
       more detailed discussion of partial and multi-segment matching, with
       examples, in the pcre2partial documentation.

       PCRE2_DFA_SHORTEST

       Setting the PCRE2_DFA_SHORTEST option causes the matching algorithm to
       stop as soon as it has found one match. Because of the way the
       alternative algorithm works, this is necessarily the shortest possible
       match at the first possible matching point in the subject string.

       PCRE2_DFA_RESTART

       When pcre2_dfa_match() returns a partial match, it is possible to call
       it again, with additional subject characters, and have it continue with
       the same match. The PCRE2_DFA_RESTART option requests this action; when
       it is set, the workspace and wscount options must reference the same
       vector as before because data about the match so far is left in them
       after a partial match. There is more discussion of this facility in the
       pcre2partial documentation.

   Successful returns from pcre2_dfa_match()
       When pcre2_dfa_match() succeeds, it may have matched more than one
       substring in the subject. Note, however, that all the matches from one
       run of the function start at the same point in the subject. The shorter
       matches are all initial substrings of the longer matches. For example,
       if the pattern

       <.*>

       is matched against the string

         This is <something> <something else> <something further> no more

       the three matched strings are

       <something> <something else> <something further>
       <something> <something else>
       <something>

       On success, the yield of the function is a number greater than zero,
       which is the number of matched substrings. The offsets of the
       substrings are returned in the ovector, and can be extracted by number
       in the same way as for pcre2_match(), but the numbers bear no relation
       to any capture groups that may exist in the pattern, because DFA
       matching does not support capturing.

       Calls to the convenience functions that extract substrings by name
       return the error PCRE2_ERROR_DFA_UFUNC (unsupported function) if used
       after a DFA match. The convenience functions that extract substrings by
       number never return PCRE2_ERROR_NOSUBSTRING.

       The matched strings are stored in the ovector in reverse order of
       length; that is, the longest matching string is first. If there were
       too many matches to fit into the ovector, the yield of the function is
       zero, and the vector is filled with the longest matches.

           Note

           PCRE2's "auto-possessification" optimization usually applies to
           character repeats at the end of a pattern (as well as internally).
           For example, the pattern a\d+ is compiled as if it were a\d++. For
           DFA matching, this means that only one possible match is found. If
           you really do want multiple matches in such cases, either use an
           ungreedy repeat such as a\d+? or set the PCRE2_NO_AUTO_POSSESS
           option when compiling.

   Error returns from pcre2_dfa_match()
       The pcre2_dfa_match() function returns a negative number when it fails.
       Many of the errors are the same as for pcre2_match(), as described
       above. There are in addition the following errors that are specific to
       pcre2_dfa_match():

       PCRE2_ERROR_DFA_UITEM

       This return is given if pcre2_dfa_match() encounters an item in the
       pattern that it does not support, for instance, the use of \C in a UTF
       mode or a backreference.

       PCRE2_ERROR_DFA_UCOND

       This return is given if pcre2_dfa_match() encounters a condition item
       that uses a backreference for the condition, or a test for recursion in
       a specific capture group. These are not supported.

       PCRE2_ERROR_DFA_UINVALID_UTF

       This return is given if pcre2_dfa_match() is called for a pattern that
       was compiled with PCRE2_MATCH_INVALID_UTF. This is not supported for
       DFA matching.

       PCRE2_ERROR_DFA_WSSIZE

       This return is given if pcre2_dfa_match() runs out of space in the
       workspace vector.

       PCRE2_ERROR_DFA_RECURSE

       When a recursion or subroutine call is processed, the matching function
       calls itself recursively, using private memory for the ovector and
       workspace. This error is given if the internal ovector is not large
       enough. This should be extremely rare, as a vector of size 1000 is
       used.

       PCRE2_ERROR_DFA_BADRESTART

       When pcre2_dfa_match() is called with the PCRE2_DFA_RESTART option,
       some plausibility checks are made on the contents of the workspace,
       which should contain data about the previous partial match. If any of
       these checks fail, this error is given.


SEE ALSO
       pcre2build(3), pcre2callout(3), pcre2demo(3), pcre2matching(3),
       pcre2partial(3), pcre2posix(3), pcre2sample(3), pcre2unicode(3).


AUTHOR
         Philip Hazel
         Retired from University Computing Service
         Cambridge, England.


REVISION
       Last updated: 08 December 2025
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.49-DEV                08 December 2025                    PCRE2API(3)
------------------------------------------------------------------------------


PCRE2BUILD(3)                        PCRE2                       PCRE2BUILD(3)


NAME
       pcre2build - PCRE2 build-time options


BUILDING PCRE2
       PCRE2 is distributed with a configure script that can be used to build
       the library in Unix-like environments using the Autotools applications.
       Also in the distribution are files to support building using CMake
       instead of configure. The text file README contains general information
       about building with Autotools (some of which is repeated below), and
       also has some comments about building on various operating systems. The
       files in the vms directory support building under OpenVMS. There is a
       lot more information about building PCRE2 without using Autotools
       (including information about using CMake and building "by hand") in the
       text file called NON-AUTOTOOLS-BUILD. You should consult this file as
       well as the README file if you are building in a non-Unix-like
       environment.


PCRE2 BUILD-TIME OPTIONS
       The rest of this document describes the optional features of PCRE2 that
       can be selected when the library is compiled. It assumes use of the
       configure script, where the optional features are selected or
       deselected by providing options to configure before running the make
       command. However, the same options can be selected in both Unix-like
       and non-Unix-like environments if you are using CMake instead of
       configure to build PCRE2.

       If you are not using Autotools or CMake, option selection can be done
       by editing the config.h file, or by passing parameter settings to the
       compiler, as described in NON-AUTOTOOLS-BUILD.

       The complete list of options for configure (which includes the standard
       ones such as the selection of the installation directory) can be
       obtained by running

       ./configure --help

       The following sections include descriptions of "on/off" options whose
       names begin with --enable or --disable. Because of the way that
       configure works, --enable and --disable always come in pairs, so the
       complementary option always exists as well, but as it specifies the
       default, it is not described. Options that specify values have names
       that start with --with. At the end of a configure run, a summary of the
       configuration is output.


BUILDING 8-BIT, 16-BIT AND 32-BIT LIBRARIES
       By default, a library called libpcre2-8 is built, containing functions
       that take string arguments contained in arrays of bytes, interpreted
       either as single-byte characters, or UTF-8 strings. You can also build
       two other libraries, called libpcre2-16 and libpcre2-32, which process
       strings that are contained in arrays of 16-bit and 32-bit code units,
       respectively. These can be interpreted either as single-unit characters
       or UTF-16/UTF-32 strings. To build these additional libraries, add one
       or both of the following to the configure command:

       --enable-pcre2-16
       --enable-pcre2-32

       If you do not want the 8-bit library, add

       --disable-pcre2-8

       as well. At least one of the three libraries must be built. Note that
       the POSIX wrapper is for the 8-bit library only, and that pcre2grep is
       an 8-bit program. Neither of these are built if you select only the
       16-bit or 32-bit libraries.


BUILDING SHARED AND STATIC LIBRARIES
       The Autotools PCRE2 building process uses libtool to build both shared
       and static libraries by default. You can suppress an unwanted library
       by adding one of

       --disable-shared
       --disable-static

       to the configure command. Setting --disable-shared ensures that PCRE2
       libraries are built as static libraries. The binaries that are then
       created as part of the build process (for example, pcre2test and
       pcre2grep) are linked statically with one or more PCRE2 libraries, but
       may also be dynamically linked with other libraries such as libc. If
       you want these binaries to be fully statically linked, you can set
       LDFLAGS like this:

       LDFLAGS=--static ./configure --disable-shared

       Note the two hyphens in --static. Of course, this works only if static
       versions of all the relevant libraries are available for linking.


UNICODE AND UTF SUPPORT
       By default, PCRE2 is built with support for Unicode and UTF character
       strings. To build it without Unicode support, add

       --disable-unicode

       to the configure command. This setting applies to all three libraries.
       It is not possible to build one library with Unicode support and
       another without in the same configuration.

       Of itself, Unicode support does not make PCRE2 treat strings as UTF-8,
       UTF-16 or UTF-32. To do that, applications that use the library can set
       the PCRE2_UTF option when they call pcre2_compile() to compile a
       pattern. Alternatively, patterns may be started with (*UTF) unless the
       application has locked this out by setting PCRE2_NEVER_UTF.

       UTF support allows the libraries to process character code points up to
       0x10ffff in the strings that they handle. Unicode support also gives
       access to the Unicode properties of characters, using pattern escapes
       such as \P, \p, and \X. Only the general category properties such as Lu
       and Nd, script names, and some bi-directional and binary properties are
       supported. Details are given in the pcre2pattern documentation.

       Pattern escapes such as \d and \w do not by default make use of Unicode
       properties. The application can request that they do by setting the
       PCRE2_UCP option. Unless the application has set PCRE2_NEVER_UCP, a
       pattern may also request this by starting with (*UCP).


DISABLING THE USE OF \C
       The \C escape sequence, which matches a single code unit, even in a UTF
       mode, can cause unpredictable behaviour because it may leave the
       current matching point in the middle of a multi-code-unit character.
       The application can lock it out by setting the PCRE2_NEVER_BACKSLASH_C
       option when calling pcre2_compile(). There is also a build-time option

       --enable-never-backslash-C

       (note the upper case C) which locks out the use of \C entirely.


JUST-IN-TIME COMPILER SUPPORT
       Just-in-time (JIT) compiler support is included in the build by
       specifying

       --enable-jit

       This support is available only for certain hardware architectures. If
       this option is set for an unsupported architecture, a building error
       occurs. If in doubt, use

       --enable-jit=auto

       which enables JIT only if the current hardware is supported. You can
       check if JIT is enabled in the configuration summary that is output at
       the end of a configure run. If you are enabling JIT under SELinux you
       may also want to add

       --enable-jit-sealloc

       which enables the use of an execmem allocator in JIT that is compatible
       with SELinux. This has no effect if JIT is not enabled. See the
       pcre2jit documentation for a discussion of JIT usage. When JIT support
       is enabled, pcre2grep automatically makes use of it, unless you add

       --disable-pcre2grep-jit

       to the configure command.


NEWLINE RECOGNITION
       By default, PCRE2 interprets the linefeed (LF) character as indicating
       the end of a line. This is the normal newline character on Unix-like
       systems. You can compile PCRE2 to use carriage return (CR) instead, by
       adding

       --enable-newline-is-cr

       to the configure command. There is also an --enable-newline-is-lf
       option, which explicitly specifies linefeed as the newline character.

       Alternatively, you can specify that line endings are to be indicated by
       the two-character sequence CRLF (CR immediately followed by LF). If you
       want this, add

       --enable-newline-is-crlf

       to the configure command. There is a fourth option, specified by

       --enable-newline-is-anycrlf

       which causes PCRE2 to recognize any of the three sequences CR, LF, or
       CRLF as indicating a line ending. A fifth option, specified by

       --enable-newline-is-any

       causes PCRE2 to recognize any Unicode newline sequence. The Unicode
       newline sequences are the three just mentioned, plus the single
       characters VT (vertical tab, U+000B), FF (form feed, U+000C), NEL (next
       line, U+0085), LS (line separator, U+2028), and PS (paragraph
       separator, U+2029). The final option is

       --enable-newline-is-nul

       which causes NUL (binary zero) to be set as the default line-ending
       character.

       Whatever default line ending convention is selected when PCRE2 is built
       can be overridden by applications that use the library. At build time
       it is recommended to use the standard for your operating system.


WHAT \R MATCHES
       By default, the sequence \R in a pattern matches any Unicode newline
       sequence, independently of what has been selected as the line ending
       sequence. If you specify

       --enable-bsr-anycrlf

       the default is changed so that \R matches only CR, LF, or CRLF.
       Whatever is selected when PCRE2 is built can be overridden by
       applications that use the library.


HANDLING VERY LARGE PATTERNS
       Within a compiled pattern, offset values are used to point from one
       part to another (for example, from an opening parenthesis to an
       alternation metacharacter). By default, in the 8-bit and 16-bit
       libraries, two-byte values are used for these offsets, leading to a
       maximum size for a compiled pattern of around 64 thousand code units.
       This is sufficient to handle all but the most gigantic patterns.
       Nevertheless, some people do want to process truly enormous patterns,
       so it is possible to compile PCRE2 to use three-byte or four-byte
       offsets by adding a setting such as

       --with-link-size=3

       to the configure command. The value given must be 2, 3, or 4. For the
       16-bit library, a value of 3 is rounded up to 4. In these libraries,
       using longer offsets slows down the operation of PCRE2 because it has
       to load additional data when handling them. For the 32-bit library the
       value is always 4 and cannot be overridden; the value of
       --with-link-size is ignored.


LIMITING PCRE2 RESOURCE USAGE
       The pcre2_match() function increments a counter each time it goes round
       its main loop. Putting a limit on this counter controls the amount of
       computing resource used by a single call to pcre2_match(). The limit
       can be changed at run time, as described in the pcre2api documentation.
       The default is 10 million, but this can be changed by adding a setting
       such as

       --with-match-limit=500000

       to the configure command. This setting also applies to the
       pcre2_dfa_match() matching function, and to JIT matching (though the
       counting is done differently).

       The pcre2_match() function uses heap memory to record backtracking
       points. The more nested backtracking points there are (that is, the
       deeper the search tree), the more memory is needed. There is an upper
       limit, specified in kibibytes (units of 1024 bytes). This limit can be
       changed at run time, as described in the pcre2api documentation. The
       default limit (in effect unlimited) is 20 million. You can change this
       by a setting such as

       --with-heap-limit=500

       which limits the amount of heap to 500 KiB. This limit applies only to
       interpretive matching in pcre2_match() and pcre2_dfa_match(), which may
       also use the heap for internal workspace when processing complicated
       patterns. This limit does not apply when JIT (which has its own memory
       arrangements) is used.

       You can also explicitly limit the depth of nested backtracking in the
       pcre2_match() interpreter. This limit defaults to the value that is set
       for --with-match-limit. You can set a lower default limit by adding,
       for example,

       --with-match-limit-depth=10000

       to the configure command. This value can be overridden at run time.
       This depth limit indirectly limits the amount of heap memory that is
       used, but because the size of each backtracking "frame" depends on the
       number of capturing parentheses in a pattern, the amount of heap that
       is used before the limit is reached varies from pattern to pattern.
       This limit was more useful in versions before 10.30, where function
       recursion was used for backtracking.

       As well as applying to pcre2_match(), the depth limit also controls the
       depth of recursive function calls in pcre2_dfa_match(). These are used
       for lookaround assertions, atomic groups, and recursion within
       patterns. The limit does not apply to JIT matching.


LIMITING VARIABLE-LENGTH LOOKBEHIND ASSERTIONS
       Lookbehind assertions in which one or more branches can match a
       variable number of characters are supported only if there is a maximum
       matching length for each top-level branch. There is a limit to this
       maximum that defaults to 255 characters. You can alter this default by
       a setting such as

       --with-max-varlookbehind=100

       The limit can be changed at runtime by calling
       pcre2_set_max_varlookbehind(). Lookbehind assertions in which every
       branch matches a fixed number of characters (not necessarily all the
       same) are not constrained by this limit.


CREATING CHARACTER TABLES AT BUILD TIME
       PCRE2 uses fixed tables for processing characters whose code points are
       less than 256. By default, PCRE2 is built with a set of tables that are
       distributed in the file src/pcre2_chartables.c.dist. These tables are
       for ASCII codes only. If you add

       --enable-rebuild-chartables

       to the configure command, the distributed tables are no longer used.
       Instead, a program called pcre2_dftables is compiled and run. This
       outputs the source for new set of tables, created in the default locale
       of your C run-time system. This method of replacing the tables does not
       work if you are cross compiling, because pcre2_dftables needs to be run
       on the local host and therefore not compiled with the cross compiler.

       If you need to create alternative tables when cross compiling, you will
       have to do so "by hand". There may also be other reasons for creating
       tables manually. To cause pcre2_dftables to be built on the local host,
       run a normal compiling command, and then run the program with the
       output file as its argument, for example:

       cc src/pcre2_dftables.c -o pcre2_dftables
       ./pcre2_dftables src/pcre2_chartables.c

       This builds the tables in the default locale of the local host. If you
       want to specify a locale, you must use the -L option:

       LC_ALL=fr_FR ./pcre2_dftables -L src/pcre2_chartables.c

       You can also specify -b (with or without -L). This causes the tables to
       be written in binary instead of as source code. A set of binary tables
       can be loaded into memory by an application and passed to
       pcre2_compile() in the same way as tables created by calling
       pcre2_maketables(). The tables are just a string of bytes, independent
       of hardware characteristics such as endianness. This means they can be
       bundled with an application that runs in different environments, to
       ensure consistent behaviour.


USING EBCDIC CODE
       PCRE2 assumes by default that it will run in an environment where the
       character code is ASCII or Unicode, which is a superset of ASCII. This
       is the case for most computer operating systems. PCRE2 can, however, be
       compiled to run in an 8-bit EBCDIC environment by adding

       --enable-ebcdic --disable-unicode

       to the configure command. You should only use it if you know that you
       are in an EBCDIC environment (for example, an IBM mainframe operating
       system).

       This setting implies --enable-rebuild-chartables, in order to ensure
       that you have the correct default character tables for your system's
       codepage. There is an exception when you set
       --enable-ebcdic-ignoring-compiler (see below), which allows using a
       default set of EBCDIC 1047 character tables rather than forcing use of
       --enable-rebuild-chartables.

       It is not supported to enable both EBCDIC input and either ASCII or
       UTF-8/16/32 in the same build of the library. When PCRE2 is built with
       EBCDIC support, it always operates in EBCDIC, and consequently
       --enable-unicode and --enable-ebcdic are mutually exclusive.

       The EBCDIC character that corresponds to an ASCII LF is assumed to have
       the value 0x15 by default. However, in some EBCDIC environments, 0x25
       is used. In such an environment you should use

       --enable-ebcdic-nl25

       (which implies --enable-ebcdic). The EBCDIC character for CR has the
       same value as in ASCII, namely, 0x0d. Whichever of 0x15 and 0x25 is not
       chosen as LF is made to correspond to the Unicode NEL character (which,
       in Unicode, is 0x85).

       The options that select newline behaviour, such as
       --enable-newline-is-cr, and equivalent run-time options, refer to these
       character values in an EBCDIC environment.

       On systems requiring an EBCDIC build of PCRE2, the compiler should be
       set to use the correct codepage, so that C character literals such as
       'z' use the correct numeric value for whichever EBCDIC codpage is in
       use. (PCRE2 cannot support multiple EBCDIC codepages dynamically.)
       However, if this not possible, then you can use

       --enable-ebcdic-ignoring-compiler

       in order to disregard the compiler's codepage, and instead force PCRE2
       to use numeric constants corresponding to the EBCDIC 1047 codepage
       instead. This can be used to build (or test) EBCDIC support on an
       ASCII/UTF-8 system such as Linux.


PCRE2GREP SUPPORT FOR EXTERNAL SCRIPTS
       By default pcre2grep supports the use of callouts with string arguments
       within the patterns it is matching. There are two kinds: one that
       generates output using local code, and another that calls an external
       program or script. If --disable-pcre2grep-callout-fork is added to the
       configure command, only the first kind of callout is supported; if
       --disable-pcre2grep-callout is used, all callouts are completely
       ignored. For more details of pcre2grep callouts, see the pcre2grep
       documentation.


PCRE2GREP OPTIONS FOR COMPRESSED FILE SUPPORT
       By default, pcre2grep reads all files as plain text. You can build it
       so that it recognizes files whose names end in .gz or .bz2, and reads
       them with libz or libbz2, respectively, by adding one or both of

       --enable-pcre2grep-libz
       --enable-pcre2grep-libbz2

       to the configure command. These options naturally require that the
       relevant libraries are installed on your system. Configuration will
       fail if they are not.


PCRE2GREP BUFFER SIZE
       pcre2grep uses an internal buffer to hold a "window" on the file it is
       scanning, in order to be able to output "before" and "after" lines when
       it finds a match. The default starting size of the buffer is 20KiB. The
       buffer itself is three times this size, but because of the way it is
       used for holding "before" lines, the longest line that is guaranteed to
       be processable is the notional buffer size. If a longer line is
       encountered, pcre2grep automatically expands the buffer, up to a
       specified maximum size, whose default is 1MiB or the starting size,
       whichever is the larger. You can change the default parameter values by
       adding, for example,

       --with-pcre2grep-bufsize=51200
       --with-pcre2grep-max-bufsize=2097152

       to the configure command. The caller of pcre2grep can override these
       values by using --buffer-size and --max-buffer-size on the command
       line.


pcre2test OPTION FOR LIBREADLINE SUPPORT
       If you add one of

       --enable-pcre2test-libreadline
       --enable-pcre2test-libedit

       to the configure command, pcre2test is linked with the libreadline or
       libedit library, respectively, and when its input is from a terminal,
       it reads it using the readline() function. This provides line-editing
       and history facilities. Note that libreadline is GPL-licensed, so if
       you distribute a binary of pcre2test linked in this way, there may be
       licensing issues. These can be avoided by linking instead with libedit,
       which has a BSD licence.

       Setting --enable-pcre2test-libreadline causes the -lreadline option to
       be added to the pcre2test build. In many operating environments with a
       system-installed readline library this is sufficient. However, in some
       environments (e.g. if an unmodified distribution version of readline is
       in use), some extra configuration may be necessary. The INSTALL file
       for libreadline says this:

         "Readline uses the termcap functions, but does not link with
         the termcap or curses library itself, allowing applications
         which link with readline the to choose an appropriate library."

       If your environment has not been set up so that an appropriate library
       is automatically included, you may need to add something like

       LIBS="-lncurses"

       immediately before the configure command.


INCLUDING DEBUGGING CODE
       If you add

       --enable-debug

       to the configure command, additional debugging code is included in the
       build. This feature is intended for use by the PCRE2 maintainers.


DEBUGGING WITH VALGRIND SUPPORT
       If you add

       --enable-valgrind

       to the configure command, PCRE2 will use valgrind annotations to mark
       certain memory regions as unaddressable. This allows it to detect
       invalid memory accesses, and is mostly useful for debugging PCRE2
       itself.


CODE COVERAGE REPORTING
       If your C compiler is gcc, you can build a version of PCRE2 that can
       generate a code coverage report for its test suite. To enable this, you
       must install lcov version 1.6 or above. Then specify

       --enable-coverage

       to the configure command and build PCRE2 in the usual way.

       Note that using ccache (a caching C compiler) is incompatible with code
       coverage reporting. If you have configured ccache to run automatically
       on your system, you must set the environment variable

       CCACHE_DISABLE=1

       before running make to build PCRE2, so that ccache is not used.

       When --enable-coverage is used, the following addition targets are
       added to the Makefile:

       make coverage

       This creates a fresh coverage report for the PCRE2 test suite. It is
       equivalent to running "make coverage-reset", "make coverage-baseline",
       "make check", and then "make coverage-report".

       make coverage-reset

       This zeroes the coverage counters, but does nothing else.

       make coverage-baseline

       This captures baseline coverage information.

       make coverage-report

       This creates the coverage report.

       make coverage-clean-report

       This removes the generated coverage report without cleaning the
       coverage data itself.

       make coverage-clean-data

       This removes the captured coverage data without removing the coverage
       files created at compile time (*.gcno).

       make coverage-clean

       This cleans all coverage data including the generated coverage report.
       For more information about code coverage, see the gcov and lcov
       documentation.


DISABLING THE Z AND T FORMATTING MODIFIERS
       The C99 standard defines formatting modifiers z and t for size_t and
       ptrdiff_t values, respectively. By default, PCRE2 uses these modifiers
       in environments other than old versions of Microsoft Visual Studio when
       STDC_VERSION is defined and has a value greater than or equal to
       199901L (indicating support for C99). However, there is at least one
       environment that claims to be C99 but does not support these modifiers.
       If

       --disable-percent-zt

       is specified, no use is made of the z or t modifiers. Instead of %td or
       %zu, a suitable format is used depending in the size of long for the
       platform.


SUPPORT FOR FUZZERS
       There is a special option for use by people who want to run fuzzing
       tests on PCRE2:

       --enable-fuzz-support

       At present this applies only to the 8-bit library. If set, it causes an
       extra library called libpcre2-fuzzsupport.a to be built, but not
       installed. This contains a single function called
       LLVMFuzzerTestOneInput() whose arguments are a pointer to a string and
       the length of the string. When called, this function tries to compile
       the string as a pattern, and if that succeeds, to match it. This is
       done both with no options and with some random options bits that are
       generated from the string.

       Setting --enable-fuzz-support also causes a binary called
       pcre2fuzzcheck to be created. This is normally run under valgrind or
       used when PCRE2 is compiled with address sanitizing enabled. It calls
       the fuzzing function and outputs information about what it is doing.
       The input strings are specified by arguments: if an argument starts
       with "=" the rest of it is a literal input string. Otherwise, it is
       assumed to be a file name, and the contents of the file are the test
       string.


OBSOLETE OPTION
       In versions of PCRE2 prior to 10.30, there were two ways of handling
       backtracking in the pcre2_match() function. The default was to use the
       system stack, but if

       --disable-stack-for-recursion

       was set, memory on the heap was used. From release 10.30 onwards this
       has changed (the stack is no longer used) and this option now does
       nothing except give a warning.


SEE ALSO
       pcre2api(3), pcre2-config(3).


AUTHOR
         Philip Hazel
         Retired from University Computing Service
         Cambridge, England.


REVISION
       Last updated: 17 October 2025
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.49-DEV                 17 October 2025                  PCRE2BUILD(3)
------------------------------------------------------------------------------


PCRE2CALLOUT(3)                      PCRE2                     PCRE2CALLOUT(3)


NAME
       pcre2callout - PCRE2 callouts


SYNOPSIS
       #include <pcre2.h>

       int (*pcre2_callout)(pcre2_callout_block *callout_block,
                            void *callout_data);

       int pcre2_callout_enumerate(
             const pcre2_code *code,
             int (*callback)(pcre2_callout_enumerate_block *callout_block,
                             void *callout_data),
             void *callout_data);


DESCRIPTION
       PCRE2 provides a feature called "callout", which is a means of
       temporarily passing control to the caller of PCRE2 in the middle of
       pattern matching. The caller of PCRE2 provides an external function by
       putting its entry point in a match context (see pcre2_set_callout() in
       the pcre2api documentation).

       When using the pcre2_substitute() function, an additional callout
       feature is available. This does a callout after each change to the
       subject string and is described in the pcre2api documentation; the rest
       of this document is concerned with callouts during pattern matching.

       Within a regular expression, (?C<arg>) indicates a point at which the
       external function is to be called. Different callout points can be
       identified by putting a number less than 256 after the letter C. The
       default value is zero. Alternatively, the argument may be a delimited
       string. The starting delimiter must be one of ` ' " ^ % # $ { and the
       ending delimiter is the same as the start, except for {, where the
       ending delimiter is }. If the ending delimiter is needed within the
       string, it must be doubled. For example, this pattern has two callout
       points:

       (?C1)abc(?C"some ""arbitrary"" text")def

       If the PCRE2_AUTO_CALLOUT option bit is set when a pattern is compiled,
       PCRE2 automatically inserts callouts, all with number 255, before each
       item in the pattern except for immediately before or after an explicit
       callout. For example, if PCRE2_AUTO_CALLOUT is used with the pattern

       A(?C3)B

       it is processed as if it were

       (?C255)A(?C3)B(?C255)

       Here is a more complicated example:

       A(\d{2}|--)

       With PCRE2_AUTO_CALLOUT, this pattern is processed as if it were

       (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)

       Notice that there is a callout before and after each parenthesis and
       alternation bar. If the pattern contains a conditional group whose
       condition is an assertion, an automatic callout is inserted immediately
       before the condition. Such a callout may also be inserted explicitly,
       for example:

       (?(?C9)(?=a)ab|de)  (?(?C%text%)(?!=d)ab|de)

       This applies only to assertion conditions (because they are themselves
       independent groups).

       Callouts can be useful for tracking the progress of pattern matching.
       The pcre2test program has a pattern qualifier (/auto_callout) that sets
       automatic callouts. When any callouts are present, the output from
       pcre2test indicates how the pattern is being matched. This is useful
       information when you are trying to optimize the performance of a
       particular pattern.


MISSING CALLOUTS
       You should be aware that, because of optimizations in the way PCRE2
       compiles and matches patterns, callouts sometimes do not happen exactly
       as you might expect.

   Auto-possessification
       At compile time, PCRE2 "auto-possessifies" repeated items when it knows
       that what follows cannot be part of the repeat. For example, a+[bc] is
       compiled as if it were a++[bc]. The pcre2test output when this pattern
       is compiled with PCRE2_ANCHORED and PCRE2_AUTO_CALLOUT and then applied
       to the string "aaaa" is:

       --->aaaa
        +0 ^        a+
        +2 ^   ^    [bc]
       No match

       This indicates that when matching [bc] fails, there is no backtracking
       into a+ (because it is being treated as a++) and therefore the callouts
       that would be taken for the backtracks do not occur. You can disable
       the auto-possessify feature by passing PCRE2_NO_AUTO_POSSESS to
       pcre2_compile(), or starting the pattern with (*NO_AUTO_POSSESS). In
       this case, the output changes to this:

       --->aaaa
        +0 ^        a+
        +2 ^   ^    [bc]
        +2 ^  ^     [bc]
        +2 ^ ^      [bc]
        +2 ^^       [bc]
       No match

       This time, when matching [bc] fails, the matcher backtracks into a+ and
       tries again, repeatedly, until a+ itself fails.

   Automatic .* anchoring
       By default, an optimization is applied when .* is the first significant
       item in a pattern. If PCRE2_DOTALL is set, so that the dot can match
       any character, the pattern is automatically anchored. If PCRE2_DOTALL
       is not set, a match can start only after an internal newline or at the
       beginning of the subject, and pcre2_compile() remembers this. If a
       pattern has more than one top-level branch, automatic anchoring occurs
       if all branches are anchorable.

       This optimization is disabled, however, if .* is in an atomic group or
       if there is a backreference to the capture group in which it appears.
       It is also disabled if the pattern contains (*PRUNE) or (*SKIP).
       However, the presence of callouts does not affect it.

       For example, if the pattern .*\d is compiled with PCRE2_AUTO_CALLOUT
       and applied to the string "aa", the pcre2test output is:

       --->aa
        +0 ^      .*
        +2 ^ ^    \d
        +2 ^^     \d
        +2 ^      \d
       No match

       This shows that all match attempts start at the beginning of the
       subject. In other words, the pattern is anchored. You can disable this
       optimization by passing PCRE2_NO_DOTSTAR_ANCHOR to pcre2_compile(), or
       starting the pattern with (*NO_DOTSTAR_ANCHOR). In this case, the
       output changes to:

       --->aa
        +0 ^      .*
        +2 ^ ^    \d
        +2 ^^     \d
        +2 ^      \d
        +0  ^     .*
        +2  ^^    \d
        +2  ^     \d
       No match

       This shows more match attempts, starting at the second subject
       character. Another optimization, described in the next section, means
       that there is no subsequent attempt to match with an empty subject.

   Other optimizations
       Other optimizations that provide fast "no match" results also affect
       callouts. For example, if the pattern is

       ab(?C4)cd

       PCRE2 knows that any matching string must contain the letter "d". If
       the subject string is "abyz", the lack of "d" means that matching
       doesn't ever start, and the callout is never reached. However, with
       "abyd", though the result is still no match, the callout is obeyed.

       For most patterns PCRE2 also knows the minimum length of a matching
       string, and will immediately give a "no match" return without actually
       running a match if the subject is not long enough, or, for unanchored
       patterns, if it has been scanned far enough.

       You can disable these optimizations by passing the
       PCRE2_NO_START_OPTIMIZE option to pcre2_compile(), or by starting the
       pattern with (*NO_START_OPT). This slows down the matching process, but
       does ensure that callouts such as the example above are obeyed.


THE CALLOUT INTERFACE
       During matching, when PCRE2 reaches a callout point, if an external
       function is provided in the match context, it is called. This applies
       to both normal, DFA, and JIT matching. The first argument to the
       callout function is a pointer to a pcre2_callout block. The second
       argument is the void * callout data that was supplied when the callout
       was set up by calling pcre2_set_callout() (see the pcre2api
       documentation). The callout block structure contains the following
       fields, not necessarily in this order:

       uint32_t      version;
       uint32_t      callout_number;
       uint32_t      capture_top;
       uint32_t      capture_last;
       uint32_t      callout_flags;
       PCRE2_SIZE   *offset_vector;
       PCRE2_SPTR    mark;
       PCRE2_SPTR    subject;
       PCRE2_SIZE    subject_length;
       PCRE2_SIZE    start_match;
       PCRE2_SIZE    current_position;
       PCRE2_SIZE    pattern_position;
       PCRE2_SIZE    next_item_length;
       PCRE2_SIZE    callout_string_offset;
       PCRE2_SIZE    callout_string_length;
       PCRE2_SPTR    callout_string;

       The version field contains the version number of the block format. The
       current version is 2; the three callout string fields were added for
       version 1, and the callout_flags field for version 2. If you are
       writing an application that might use an earlier release of PCRE2, you
       should check the version number before accessing any of these fields.
       The version number will increase in future if more fields are added,
       but the intention is never to remove any of the existing fields.

   Fields for numerical callouts
       For a numerical callout, callout_string is NULL, and callout_number
       contains the number of the callout, in the range 0-255. This is the
       number that follows (?C for callouts that part of the pattern; it is
       255 for automatically generated callouts.

   Fields for string callouts
       For callouts with string arguments, callout_number is always zero, and
       callout_string points to the string that is contained within the
       compiled pattern. Its length is given by callout_string_length.
       Duplicated ending delimiters that were present in the original pattern
       string have been turned into single characters, but there is no other
       processing of the callout string argument. An additional code unit
       containing binary zero is present after the string, but is not included
       in the length. The delimiter that was used to start the string is also
       stored within the pattern, immediately before the string itself. You
       can access this delimiter as callout_string[-1] if you need it.

       The callout_string_offset field is the code unit offset to the start of
       the callout argument string within the original pattern string. This is
       provided for the benefit of applications such as script languages that
       might need to report errors in the callout string within the pattern.

   Fields for all callouts
       The remaining fields in the callout block are the same for both kinds
       of callout.

       The offset_vector field is a pointer to a vector of capturing offsets
       (the "ovector"). You may read the elements in this vector, but you must
       not change any of them.

       For calls to pcre2_match(), the offset_vector field is not (since
       release 10.30) a pointer to the actual ovector that was passed to the
       matching function in the match data block. Instead it points to an
       internal ovector of a size large enough to hold all possible captured
       substrings in the pattern. Note that whenever a recursion or subroutine
       call within a pattern completes, the capturing state is reset to what
       it was before.

       The capture_last field contains the number of the most recently
       captured substring, and the capture_top field contains one more than
       the number of the highest numbered captured substring so far. If no
       substrings have yet been captured, the value of capture_last is 0 and
       the value of capture_top is 1. The values of these fields do not always
       differ by one; for example, when the callout in the pattern a)(b(?C2)
       is taken, capture_last is 1 but capture_top is 4.

       The contents of ovector[2] to ovector[capture_top*2-1] can be inspected
       in order to extract substrings that have been matched so far, in the
       same way as extracting substrings after a match has completed. The
       values in ovector[0] and ovector[1] are always PCRE2_UNSET because the
       match is by definition not complete. Substrings that have not been
       captured but whose numbers are less than capture_top also have both of
       their ovector slots set to PCRE2_UNSET.

       For DFA matching, the offset_vector field points to the ovector that
       was passed to the matching function in the match data block for
       callouts at the top level, but to an internal ovector during the
       processing of pattern recursions, lookarounds, and atomic groups.
       However, these ovectors hold no useful information because
       pcre2_dfa_match() does not support substring capturing. The value of
       capture_top is always 1 and the value of capture_last is always 0 for
       DFA matching.

       The subject and subject_length fields contain copies of the values that
       were passed to the matching function.

       The start_match field normally contains the offset within the subject
       at which the current match attempt started. However, if the escape
       sequence \K has been encountered, this value is changed to reflect the
       modified starting point. If the pattern is not anchored, the callout
       function may be called several times from the same point in the pattern
       for different starting points in the subject.

       The current_position field contains the offset within the subject of
       the current match pointer.

       The pattern_position field contains the offset in the pattern string to
       the next item to be matched.

       The next_item_length field contains the length of the next item to be
       processed in the pattern string. When the callout is at the end of the
       pattern, the length is zero. When the callout precedes an opening
       parenthesis, the length includes meta characters that follow the
       parenthesis. For example, in a callout before an assertion such as
       (?=ab) the length is 3. For an alternation bar or a closing
       parenthesis, the length is one, unless a closing parenthesis is
       followed by a quantifier, in which case its length is included. (This
       changed in release 10.23. In earlier releases, before an opening
       parenthesis the length was that of the entire group, and before an
       alternation bar or a closing parenthesis the length was zero.)

       The pattern_position and next_item_length fields are intended to help
       in distinguishing between different automatic callouts, which all have
       the same callout number. However, they are set for all callouts, and
       are used by pcre2test to show the next item to be matched when
       displaying callout information.

       In callouts from pcre2_match() the mark field contains a pointer to the
       zero-terminated name of the most recently passed (*MARK), (*PRUNE), or
       (*THEN) item in the match, or NULL if no such items have been passed.
       Instances of (*PRUNE) or (*THEN) without a name do not obliterate a
       previous (*MARK). In callouts from the DFA matching function this field
       always contains NULL.

       The callout_flags field is always zero in callouts from
       pcre2_dfa_match() or when JIT is being used. When pcre2_match() without
       JIT is used, the following bits may be set:

           PCRE2_CALLOUT_STARTMATCH
                                   This is set for the first callout after the
                                   start of matching for each new starting
                                   position in the subject.

           PCRE2_CALLOUT_BACKTRACK This is set if there has been a matching
                                   backtrack since the previous callout, or
                                   since the start of matching if this is the
                                   first callout from a pcre2_match() run.

       Both bits are set when a backtrack has caused a "bumpalong" to a new
       starting position in the subject. Output from pcre2test does not
       indicate the presence of these bits unless the callout_extra modifier
       is set.

       The information in the callout_flags field is provided so that
       applications can track and tell their users how matching with
       backtracking is done. This can be useful when trying to optimize
       patterns, or just to understand how PCRE2 works. There is no support in
       pcre2_dfa_match() because there is no backtracking in DFA matching, and
       there is no support in JIT because JIT is all about maximimizing
       matching performance. In both these cases the callout_flags field is
       always zero.


RETURN VALUES FROM CALLOUTS
       The external callout function returns an integer to PCRE2. If the value
       is zero, matching proceeds as normal. If the value is greater than
       zero, matching fails at the current point, but the testing of other
       matching possibilities goes ahead, just as if a lookahead assertion had
       failed. If the value is less than zero, the match is abandoned, and the
       matching function returns the negative value.

       Negative values should normally be chosen from the set of
       PCRE2_ERROR_xxx values. In particular, PCRE2_ERROR_NOMATCH forces a
       standard "no match" failure. The error number PCRE2_ERROR_CALLOUT is
       reserved for use by callout functions; it will never be used by PCRE2
       itself.


CALLOUT ENUMERATION
       int pcre2_callout_enumerate(
             const pcre2_code *code,
             int (*callback)(pcre2_callout_enumerate_block *callout_block,
                             void *callout_data),
             void *callout_data);

       A script language that supports the use of string arguments in callouts
       might like to scan all the callouts in a pattern before running the
       match. This can be done by calling pcre2_callout_enumerate(). The first
       argument is a pointer to a compiled pattern, the second points to a
       callback function, and the third is arbitrary user data. The callback
       function is called for every callout in the pattern in the order in
       which they appear. Its first argument is a pointer to a callout
       enumeration block, and its second argument is the user_data value that
       was passed to pcre2_callout_enumerate(). The data block contains the
       following fields:

           version               Block version number
           pattern_position      Offset to next item in pattern
           next_item_length      Length of next item in pattern
           callout_number        Number for numbered callouts
           callout_string_offset Offset to string within pattern
           callout_string_length Length of callout string
           callout_string        Points to callout string or is NULL

       The version number is currently 0. It will increase if new fields are
       ever added to the block. The remaining fields are the same as their
       namesakes in the pcre2_callout block that is used for callouts during
       matching, as described above.

       Note that the value of pattern_position is unique for each callout.
       However, if a callout occurs inside a group that is quantified with a
       non-zero minimum or a fixed maximum, the group is replicated inside the
       compiled pattern. For example, a pattern such as /(a){2}/ is compiled
       as if it were /(a)(a)/. This means that the callout will be enumerated
       more than once, but with the same value for pattern_position in each
       case.

       The callback function should normally return zero. If it returns a
       non-zero value, scanning the pattern stops, and that value is returned
       from pcre2_callout_enumerate().


AUTHOR
         Philip Hazel
         Retired from University Computing Service
         Cambridge, England.


REVISION
       Last updated: 26 February 2025
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.49-DEV                26 February 2025                PCRE2CALLOUT(3)
------------------------------------------------------------------------------


PCRE2COMPAT(3)                       PCRE2                      PCRE2COMPAT(3)


NAME
       pcre2compat - PCRE2 compatibility


DIFFERENCES BETWEEN PCRE2 AND PERL
       This document describes some of the known differences in the ways that
       PCRE2 and Perl handle regular expressions. The differences described
       here are with respect to Perl version 5.38.0, but as both Perl and
       PCRE2 are continually changing, the information may at times be out of
       date.

        1. When PCRE2_DOTALL (equivalent to Perl's /s qualifier) is not set,
           the behaviour of the . metacharacter differs from Perl. In PCRE2, .
           matches the next character unless it is the start of a newline
           sequence. This means that, if the newline setting is CR, CRLF, or
           NUL, '.' will match the code point LF (0x0A) in ASCII/Unicode
           environments, and NL (either 0x15 or 0x25) when using EBCDIC. In
           Perl, '.' appears never to match LF, even when 0x0A is not a
           newline indicator.

        2. PCRE2 has only a subset of Perl's Unicode support. Details of what
           it does have are given in the pcre2unicode page.

        3. Like Perl, PCRE2 allows repeat quantifiers on parenthesized
           assertions, but they do not mean what you might think. For example,
           (?!a){3} does not assert that the next three characters are not
           "a". It just asserts that the next character is not "a" three times
           (in principle; PCRE2 optimizes this to run the assertion just
           once). Perl allows some repeat quantifiers on other assertions, for
           example, \b* , but these do not seem to have any use. PCRE2 does
           not allow any kind of quantifier on non-lookaround assertions.

        4. If a braced quantifier such as {1,2} appears where there is nothing
           to repeat (for example, at the start of a branch), PCRE2 raises an
           error whereas Perl treats the quantifier characters as literal.
           When a braced quantifier (...){min,max} has min > max, Perl treats
           it as an item which fails to match any portion of the subject (as
           no number of repetitions can meet the condition), and additionally
           issues a warning when in warning mode. PCRE2 has no warning
           features, so it gives an error in this case.

        5. Capture groups that occur inside negative lookaround assertions are
           counted, but their entries in the offsets vector are set only when
           a negative assertion is a condition that has a matching branch
           (that is, the condition is false). Perl may set such capture groups
           in other circumstances.

        6. The following Perl escape sequences are not supported: \F, \l, \L,
           \u, \U, and \N when followed by a character name. \N on its own,
           matching a non-newline character, and \N{U+dd..}, matching a
           Unicode code point, are supported. The escapes that modify the case
           of following letters are implemented by Perl's general
           string-handling and are not part of its pattern matching engine. If
           any of these are encountered by PCRE2, an error is generated by
           default. However, if either of the PCRE2_ALT_BSUX or
           PCRE2_EXTRA_ALT_BSUX options is set, \U and \u are interpreted as
           ECMAScript interprets them.

        7. The Perl escape sequences \p, \P, and \X are supported only if
           PCRE2 is built with Unicode support (the default). The properties
           that can be tested with \p and \P are limited to the general
           category properties such as Lu and Nd, the derived properties Any
           and Lc (synonym L&), script names such as Greek or Han, Bidi_Class,
           Bidi_Control, and a few binary properties. Both PCRE2 and Perl
           support the Cs (surrogate) property, but in PCRE2 its use is
           limited. See the pcre2pattern documentation for details. The long
           synonyms for property names that Perl supports (such as \p{Letter})
           are not supported by PCRE2, nor is it permitted to prefix any of
           these properties with "Is".

        8. PCRE2 supports the \Q...\E escape for quoting substrings.
           Characters in between are treated as literals. However, this is
           slightly different from Perl in that $ and @ are also handled as
           literals inside the quotes. In Perl, they cause variable
           interpolation (PCRE2 does not have variables). Also, Perl does
           "double-quotish backslash interpolation" on any backslashes between
           \Q and \E which, its documentation says, "may lead to confusing
           results". PCRE2 treats a backslash between \Q and \E just like any
           other character. Note the following examples:

           Pattern            PCRE2 matches     Perl matches

           \Qabc$xyz\E        abc$xyz           abc followed by the
                                                  contents of $xyz
           \Qabc\$xyz\E       abc\$xyz          abc\$xyz
           \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz
           \QA\B\E            A\B               A\B
           \Q\\E              \                 \\E

       The \Q...\E sequence is recognized both inside and outside character
       classes by both PCRE2 and Perl. Another difference from Perl is that
       any appearance of \Q or \E inside what might otherwise be a quantifier
       causes PCRE2 not to recognize the sequence as a quantifier. Perl
       recognizes a quantifier if (redundantly) either of the numbers is
       inside \Q...\E, but not if the separating comma is. When not recognized
       as a quantifier a sequence such as {\Q1\E,2} is treated as the literal
       string "{1,2}".

        1. Fairly obviously, PCRE2 does not support the (?{code}) and
           (??{code}) constructions. However, PCRE2 does have a "callout"
           feature, which allows an external function to be called during
           pattern matching. See the pcre2callout documentation for details.

        2. Subroutine calls (whether recursive or not) were treated as atomic
           groups up to PCRE2 release 10.23, but from release 10.30 this
           changed, and backtracking into subroutine calls is now supported,
           as in Perl.

        3. In PCRE2, if any of the backtracking control verbs are used in a
           group that is called as a subroutine (whether or not recursively),
           their effect is confined to that group; it does not extend to the
           surrounding pattern. This is not always the case in Perl. In
           particular, if (*THEN) is present in a group that is called as a
           subroutine, its action is limited to that group, even if the group
           does not contain any | characters. Note that such groups are
           processed as anchored at the point where they are tested. PCRE2
           also confines all control verbs within atomic assertions, again
           including (*THEN) in assertions with only one branch.

        4. If a pattern contains more than one backtracking control verb, the
           first one that is backtracked onto acts. For example, in the
           pattern A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but
           a failure in C triggers (*PRUNE). Perl's behaviour is more complex;
           in many cases it is the same as PCRE2, but there are cases where it
           differs.

        5. There are some differences that are concerned with the settings of
           captured strings when part of a pattern is repeated. For example,
           matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
           unset, but in PCRE2 it is set to "b".

        6. PCRE2's handling of duplicate capture group numbers and names is
           not as general as Perl's. This is a consequence of the fact the
           PCRE2 works internally just with numbers, using an external table
           to translate between numbers and names. In particular, a pattern
           such as (?|(?<a>A)|(?<b>B)), where the two capture groups have the
           same number but different names, is not supported, and causes an
           error at compile time. If it were allowed, it would not be possible
           to distinguish which group matched, because both names map to
           capture group number 1. To avoid this confusing situation, an error
           is given at compile time.

        7. Perl used to recognize comments in some places that PCRE2 does not,
           for example, between the ( and ? at the start of a group. If the /x
           modifier is set, Perl allowed white space between ( and ? though
           the latest Perls give an error (for a while it was just
           deprecated). There may still be some cases where Perl behaves
           differently.

        8. Perl, when in warning mode, gives warnings for character classes
           such as [A-\d] or [a-[:digit:]]. It then treats the hyphens as
           literals. PCRE2 has no warning features, so it gives an error in
           these cases because they are almost certainly user mistakes.

        9. In PCRE2, until release 10.45, the upper/lower case character
           properties Lu and Ll were not affected when case-independent
           matching was specified. Perl has changed in this respect, and PCRE2
           has now changed to match. When caseless matching is in force, Lu,
           Ll, and Lt (title case) are all treated as Lc (cased letter).

        10. From release 5.32.0, Perl locks out the use of \K in lookaround
           assertions. From release 10.38 PCRE2 does the same by default.
           However, there is an option for re-enabling the previous behaviour.
           When this option is set, \K is acted on when it occurs in positive
           assertions, but is ignored in negative assertions.

        11. PCRE2 provides some extensions to the Perl regular expression
           facilities. Perl 5.10 included new features that were not in
           earlier versions of Perl, some of which (such as named parentheses)
           were in PCRE2 for some time before. This list is with respect to
           Perl 5.38:

       (a) If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set, the
       $ meta-character matches only at the very end of the string.

       (b) A backslash followed by a letter with no special meaning is
       faulted. (Perl can be made to issue a warning.)

       (c) If PCRE2_UNGREEDY is set, the greediness of the repetition
       quantifiers is inverted, that is, by default they are not greedy, but
       if followed by a question mark they are.

       (d) PCRE2_ANCHORED can be used at matching time to force a pattern to
       be tried only at the first matching position in the subject string.

       (e) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY and
       PCRE2_NOTEMPTY_ATSTART options have no Perl equivalents.

       (f) The \R escape sequence can be restricted to match only CR, LF, or
       CRLF by the PCRE2_BSR_ANYCRLF option.

       (g) The callout facility is PCRE2-specific. Perl supports codeblocks
       and variable interpolation, but not general hooks on every match.

       (h) The partial matching facility is PCRE2-specific.

       (i) The alternative matching function (pcre2_dfa_match()) matches in a
       different way and is not Perl-compatible.

       (j) PCRE2 recognizes some special sequences such as (*CR) or (*NO_JIT)
       at the start of a pattern. These set overall options that cannot be
       changed within the pattern.

       (k) PCRE2 supports non-atomic positive lookaround assertions. This is
       an extension to the lookaround facilities. The default, Perl-compatible
       lookarounds are atomic.

       (l) There are three syntactical items in patterns that can refer to a
       capturing group by number: back references such as \g{2}, subroutine
       calls such as (?3), and condition references such as (?(4)...). PCRE2
       supports relative group numbers such as +2 and -4 in all three cases.
       Perl supports both plus and minus for subroutine calls, but only minus
       for back references, and no relative numbering at all for conditions.

       (m) The scan substring assertion (syntax (*scs:(n)...) is a PCRE2
       extension that is not available in Perl.

        1. Perl has different limits than PCRE2. See the pcre2limits
           documentation for details. Perl went with 5.10 from recursion to
           iteration keeping the intermediate matches on the heap, which is
           ~10% slower but does not fall into any stack-overflow limit. PCRE2
           made a similar change at release 10.30, and also has many
           build-time and run-time customizable limits.

        2. Unlike Perl, PCRE2 doesn't have character set modifiers and
           specially no way to set characters by context just like Perl's
           "/d". A regular expression using PCRE2_UTF and PCRE2_UCP will use
           similar rules to Perl's "/u"; something closer to "/a" could be
           selected by adding other PCRE2_EXTRA_ASCII* options on top.

        3. Some recursive patterns that Perl diagnoses as infinite recursions
           can be handled by PCRE2, either by the interpreter or the JIT. An
           example is /(?:|(?0)abcd)(?(R)|\z)/, which matches a sequence of
           any number of repeated "abcd" substrings at the end of the subject.

        4. Both PCRE2 and Perl error when \x{ escapes are invalid, but Perl
           tries to recover and prints a warning if the problem was that an
           invalid hexadecimal digit was found. Since PCRE2 doesn't have
           warnings it returns an error instead. Additionally, Perl accepts
           \x{} and generates NUL unlike PCRE2.

        5. From release 10.45, PCRE2 gives an error if \x is not followed by a
           hexadecimal digit or a curly bracket. It used to interpret this as
           the NUL character. Perl still generates NUL, but warns when in
           warning mode in most cases.


AUTHOR
         Philip Hazel
         Retired from University Computing Service
         Cambridge, England.


REVISION
       Last updated: 02 June 2025
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.49-DEV                  02 June 2025                   PCRE2COMPAT(3)
------------------------------------------------------------------------------


PCRE2JIT(3)                          PCRE2                         PCRE2JIT(3)


NAME
       pcre2jit - PCRE2 just-in-time compilation support


PCRE2 JUST-IN-TIME COMPILER SUPPORT
       Just-in-time compiling is a heavyweight optimization that can greatly
       speed up pattern matching. However, it comes at the cost of extra
       processing before the match is performed, so it is of most benefit when
       the same pattern is going to be matched many times. This does not
       necessarily mean many calls of a matching function; if the pattern is
       not anchored, matching attempts may take place many times at various
       positions in the subject, even for a single call. Therefore, if the
       subject string is very long, it may still pay to use JIT even for
       one-off matches. JIT support is available for all of the 8-bit, 16-bit
       and 32-bit PCRE2 libraries.

       JIT support applies only to the traditional Perl-compatible matching
       function. It does not apply when the DFA matching function is being
       used. The code for JIT support was written by Zoltan Herczeg.


AVAILABILITY OF JIT SUPPORT
       JIT support is an optional feature of PCRE2. The "configure" option
       --enable-jit (or equivalent CMake option) must be set when PCRE2 is
       built if you want to use JIT. The support is limited to the following
       hardware platforms:

         ARM 32-bit (v7, and Thumb2)
         ARM 64-bit
         DEC Alpha 64-bit
         IBM s390x 64 bit
         Intel x86 32-bit and 64-bit
         LoongArch 64 bit
         MIPS 32-bit and 64-bit
         Power PC 32-bit and 64-bit
         RISC-V 32-bit and 64-bit

       If --enable-jit is set on an unsupported platform, compilation fails.

       A client program can tell if JIT support has been compiled by calling
       pcre2_config() with the PCRE2_CONFIG_JIT option. The result is one if
       PCRE2 was built with JIT support, and zero otherwise. However, having
       the JIT code available does not guarantee that it will be used for any
       particular match. One reason for this is that there are a number of
       options and pattern items that are not supported by JIT (see below).
       Another reason is that in some environments JIT is unable to get
       executable memory in which to build its compiled code. The only
       guarantee from pcre2_config() is that if it returns zero, JIT will
       definitely not be used.

       As of release 10.45 there is a more informative way to test for JIT
       support. If pcre2_jit_compile() is called with the single option
       PCRE2_JIT_TEST_ALLOC it returns zero if JIT is available and has a
       working allocator. Otherwise it returns PCRE2_ERROR_NOMEMORY if JIT is
       available but cannot allocate executable memory, or
       PCRE2_ERROR_JIT_UNSUPPORTED if JIT support is not compiled. The code
       argument is ignored, so it can be a NULL value.

       A simple program does not need to check availability in order to use
       JIT when possible. The API is implemented in a way that falls back to
       the interpretive code if JIT is not available or cannot be used for a
       given match. For programs that need the best possible performance,
       there is a "fast path" API that is JIT-specific.


SIMPLE USE OF JIT
       To make use of the JIT support in the simplest way, all you have to do
       is to call pcre2_jit_compile() after successfully compiling a pattern
       with pcre2_compile(). This function has two arguments: the first is the
       compiled pattern pointer that was returned by pcre2_compile(), and the
       second is zero or more of the following option bits:
       PCRE2_JIT_COMPLETE, PCRE2_JIT_PARTIAL_HARD, or PCRE2_JIT_PARTIAL_SOFT.

       If JIT support is not available, a call to pcre2_jit_compile() does
       nothing and returns PCRE2_ERROR_JIT_BADOPTION. Otherwise, the compiled
       pattern is passed to the JIT compiler, which turns it into machine code
       that executes much faster than the normal interpretive code, but yields
       exactly the same results. The returned value from pcre2_jit_compile()
       is zero on success, or a negative error code.

       There is a limit to the size of pattern that JIT supports, imposed by
       the size of machine stack that it uses. The exact rules are not
       documented because they may change at any time, in particular, when new
       optimizations are introduced. If a pattern is too big, a call to
       pcre2_jit_compile() returns PCRE2_ERROR_NOMEMORY.

       PCRE2_JIT_COMPLETE requests the JIT compiler to generate code for
       complete matches. If you want to run partial matches using the
       PCRE2_PARTIAL_HARD or PCRE2_PARTIAL_SOFT options of pcre2_match(), you
       should set one or both of the other options as well as, or instead of
       PCRE2_JIT_COMPLETE. The JIT compiler generates different optimized code
       for each of the three modes (normal, soft partial, hard partial). When
       pcre2_match() is called, the appropriate code is run if it is
       available. Otherwise, the pattern is matched using interpretive code.

       You can call pcre2_jit_compile() multiple times for the same compiled
       pattern. It does nothing if it has previously compiled code for any of
       the option bits. For example, you can call it once with
       PCRE2_JIT_COMPLETE and (perhaps later, when you find you need partial
       matching) again with PCRE2_JIT_COMPLETE and PCRE2_JIT_PARTIAL_HARD.
       This time it will ignore PCRE2_JIT_COMPLETE and just compile code for
       partial matching. If pcre2_jit_compile() is called with no option bits
       set, it immediately returns zero. This is an alternative way of testing
       whether JIT support has been compiled.

       At present, it is not possible to free JIT compiled code except when
       the entire compiled pattern is freed by calling pcre2_code_free().

       In some circumstances you may need to call additional functions. These
       are described in the section entitled "Controlling the JIT stack"
       below.

       There are some pcre2_match() options that are not supported by JIT, and
       there are also some pattern items that JIT cannot handle. Details are
       given below. In both cases, matching automatically falls back to the
       interpretive code. If you want to know whether JIT was actually used
       for a particular match, you should arrange for a JIT callback function
       to be set up as described in the section entitled "Controlling the JIT
       stack" below, even if you do not need to supply a non-default JIT
       stack. Such a callback function is called whenever JIT code is about to
       be obeyed. If the match-time options are not right for JIT execution,
       the callback function is not obeyed.

       If the JIT compiler finds an unsupported item, no JIT data is
       generated. You can find out if JIT compilation was successful for a
       compiled pattern by calling pcre2_pattern_info() with the
       PCRE2_INFO_JITSIZE option. A non-zero result means that JIT compilation
       was successful. A result of 0 means that JIT support is not available,
       or the pattern was not processed by pcre2_jit_compile(), or the JIT
       compiler was not able to handle the pattern. Successful JIT compilation
       does not, however, guarantee the use of JIT at match time because there
       are some match time options that are not supported by JIT.


MATCHING SUBJECTS CONTAINING INVALID UTF
       When a pattern is compiled with the PCRE2_UTF option, subject strings
       are normally expected to be a valid sequence of UTF code units. By
       default, this is checked at the start of matching and an error is
       generated if invalid UTF is detected. The PCRE2_NO_UTF_CHECK option can
       be passed to pcre2_match() to skip the check (for improved performance)
       if you are sure that a subject string is valid. If this option is used
       with an invalid string, the result is undefined. The calling program
       may crash or loop or otherwise misbehave.

       However, a way of running matches on strings that may contain invalid
       UTF sequences is available. Calling pcre2_compile() with the
       PCRE2_MATCH_INVALID_UTF option has two effects: it tells the
       interpreter in pcre2_match() to support invalid UTF, and, if
       pcre2_jit_compile() is subsequently called, the compiled JIT code also
       supports invalid UTF. Details of how this support works, in both the
       JIT and the interpretive cases, is given in the pcre2unicode
       documentation.

       There is also an obsolete option for pcre2_jit_compile() called
       PCRE2_JIT_INVALID_UTF, which currently exists only for backward
       compatibility. It is superseded by the pcre2_compile() option
       PCRE2_MATCH_INVALID_UTF and should no longer be used. It may be removed
       in future.


UNSUPPORTED OPTIONS AND PATTERN ITEMS
       The pcre2_match() options that are supported for JIT matching are
       PCRE2_COPY_MATCHED_SUBJECT, PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY,
       PCRE2_NOTEMPTY_ATSTART, PCRE2_NO_UTF_CHECK, PCRE2_PARTIAL_HARD, and
       PCRE2_PARTIAL_SOFT. The PCRE2_ANCHORED and PCRE2_ENDANCHORED options
       are not supported at match time.

       If the PCRE2_NO_JIT option is passed to pcre2_match() it disables the
       use of JIT, forcing matching by the interpreter code.

       The only unsupported pattern items are \C (match a single data unit)
       when running in a UTF mode, and a callout immediately before an
       assertion condition in a conditional group.


RETURN VALUES FROM JIT MATCHING
       When a pattern is matched using JIT, the return values are the same as
       those given by the interpretive pcre2_match() code, with the addition
       of one new error code: PCRE2_ERROR_JIT_STACKLIMIT. This means that the
       memory used for the JIT stack was insufficient. See "Controlling the
       JIT stack" below for a discussion of JIT stack usage.

       The error code PCRE2_ERROR_MATCHLIMIT is returned by the JIT code if
       searching a very large pattern tree goes on for too long, as it is in
       the same circumstance when JIT is not used, but the details of exactly
       what is counted are not the same. The PCRE2_ERROR_DEPTHLIMIT error code
       is never returned when JIT matching is used.


CONTROLLING THE JIT STACK
       When the compiled JIT code runs, it needs a block of memory to use as a
       stack. By default, it uses 32KiB on the machine stack. However, some
       large or complicated patterns need more than this. The error
       PCRE2_ERROR_JIT_STACKLIMIT is given when there is not enough stack.
       Three functions are provided for managing blocks of memory for use as
       JIT stacks. There is further discussion about the use of JIT stacks in
       the section entitled "JIT stack FAQ" below.

       The pcre2_jit_stack_create() function creates a JIT stack. Its
       arguments are a starting size, a maximum size, and a general context
       (for memory allocation functions, or NULL for standard memory
       allocation). It returns a pointer to an opaque structure of type
       pcre2_jit_stack, or NULL if there is an error. The
       pcre2_jit_stack_free() function is used to free a stack that is no
       longer needed. If its argument is NULL, this function returns
       immediately, without doing anything. (For the technically minded: the
       address space is allocated by mmap or VirtualAlloc.) A maximum stack
       size of 512KiB to 1MiB should be more than enough for any pattern.

       The pcre2_jit_stack_assign() function specifies which stack JIT code
       should use. Its arguments are as follows:

       pcre2_match_context  *mcontext
       pcre2_jit_callback    callback
       void                 *data

       The first argument is a pointer to a match context. When this is
       subsequently passed to a matching function, its information determines
       which JIT stack is used. If this argument is NULL, the function returns
       immediately, without doing anything. There are three cases for the
       values of the other two options:

         (1) If callback is NULL and data is NULL, an internal 32KiB block
             on the machine stack is used. This is the default when a match
             context is created.

         (2) If callback is NULL and data is not NULL, data must be
             a pointer to a valid JIT stack, the result of calling
           pcre2_jit_stack_create().

         (3) If callback is not NULL, it must point to a function that is
             called with data as an argument at the start of matching, in
             order to set up a JIT stack. If the return from the callback
             function is NULL, the internal 32KiB stack is used; otherwise the
             return value must be a valid JIT stack, the result of calling
             pcre2_jit_stack_create().

       A callback function is obeyed whenever JIT code is about to be run; it
       is not obeyed when pcre2_match() is called with options that are
       incompatible for JIT matching. A callback function can therefore be
       used to determine whether a match operation was executed by JIT or by
       the interpreter.

       You may safely use the same JIT stack for more than one pattern (either
       by assigning directly or by callback), as long as the patterns are
       matched sequentially in the same thread. Currently, the only way to set
       up non-sequential matches in one thread is to use callouts: if a
       callout function starts another match, that match must use a different
       JIT stack to the one used for currently suspended match(es).

       In a multithread application, if you do not specify a JIT stack, or if
       you assign or pass back NULL from a callback, that is thread-safe,
       because each thread has its own machine stack. However, if you assign
       or pass back a non-NULL JIT stack, this must be a different stack for
       each thread so that the application is thread-safe.

       Strictly speaking, even more is allowed. You can assign the same
       non-NULL stack to a match context that is used by any number of
       patterns, as long as they are not used for matching by multiple threads
       at the same time. For example, you could use the same stack in all
       compiled patterns, with a global mutex in the callback to wait until
       the stack is available for use. However, this is an inefficient
       solution, and not recommended.

       This is a suggestion for how a multithreaded program that needs to set
       up non-default JIT stacks might operate:

       During thread initialization
         thread_local_var = pcre2_jit_stack_create(...)

       During thread exit
         pcre2_jit_stack_free(thread_local_var)

       Use a one-line callback function
         return thread_local_var

       All the functions described in this section do nothing if JIT is not
       available.


JIT STACK FAQ
       (1) Why do we need JIT stacks?

       PCRE2 (and JIT) is a recursive, depth-first engine, so it needs a stack
       where the local data of the current node is pushed before checking its
       child nodes. Allocating real machine stack on some platforms is
       difficult. For example, the stack chain needs to be updated every time
       if we extend the stack on PowerPC. Although it is possible, its
       updating time overhead decreases performance. So we do the recursion in
       memory.

       (2) Why don't we simply allocate blocks of memory with malloc()?

       Modern operating systems have a nice feature: they can reserve an
       address space instead of allocating memory. We can safely allocate
       memory pages inside this address space, so the stack could grow without
       moving memory data (this is important because of pointers). Thus we can
       allocate 1MiB address space, and use only a single memory page (usually
       4KiB) if that is enough. However, we can still grow up to 1MiB anytime
       if needed.

       (3) Who "owns" a JIT stack?

       The owner of the stack is the user program, not the JIT studied pattern
       or anything else. The user program must ensure that if a stack is being
       used by pcre2_match(), (that is, it is assigned to a match context that
       is passed to the pattern currently running), that stack must not be
       used by any other threads (to avoid overwriting the same memory area).
       The best practice for multithreaded programs is to allocate a stack for
       each thread, and return this stack through the JIT callback function.

       (4) When should a JIT stack be freed?

       You can free a JIT stack at any time, as long as it will not be used by
       pcre2_match() again. When you assign the stack to a match context, only
       a pointer is set. There is no reference counting or any other magic.
       You can free compiled patterns, contexts, and stacks in any order,
       anytime. Just do not call pcre2_match() with a match context pointing
       to an already freed stack, as that will cause SEGFAULT. (Also, do not
       free a stack currently used by pcre2_match() in another thread). You
       can also replace the stack in a context at any time when it is not in
       use. You should free the previous stack before assigning a replacement.

       (5) Should I allocate/free a stack every time before/after calling
       pcre2_match()?

       No, because this is too costly in terms of resources. However, you
       could implement some clever idea which release the stack if it is not
       used in let's say two minutes. The JIT callback can help to achieve
       this without keeping a list of patterns.

       (6) OK, the stack is for long term memory allocation. But what happens
       if a pattern causes stack overflow with a stack of 1MiB? Is that 1MiB
       kept until the stack is freed?

       Especially on embedded systems, it might be a good idea to release
       memory sometimes without freeing the stack. There is no API for this at
       the moment. Probably a function call which returns with the currently
       allocated memory for any stack and another which allows releasing
       memory (shrinking the stack) would be a good idea if someone needs
       this.

       (7) This is too much of a headache. Isn't there any better solution for
       JIT stack handling?

       No, thanks to Windows. If POSIX threads were used everywhere, we could
       throw out this complicated API.


FREEING JIT SPECULATIVE MEMORY
       void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);

       The JIT executable allocator does not free all memory when it is
       possible. It expects new allocations, and keeps some free memory around
       to improve allocation speed. However, in low memory conditions, it
       might be better to free all possible memory. You can cause this to
       happen by calling pcre2_jit_free_unused_memory(). Its argument is a
       general context, for custom memory management, or NULL for standard
       memory management.


EXAMPLE CODE
       This is a single-threaded example that specifies a JIT stack without
       using a callback. A real program should include error checking after
       all the function calls.

       int rc;
       pcre2_code *re;
       pcre2_match_data *match_data;
       pcre2_match_context *mcontext;
       pcre2_jit_stack *jit_stack;

       re = pcre2_compile(pattern, PCRE2_ZERO_TERMINATED, 0,
         &errornumber, &erroffset, NULL);
       rc = pcre2_jit_compile(re, PCRE2_JIT_COMPLETE);
       mcontext = pcre2_match_context_create(NULL);
       jit_stack = pcre2_jit_stack_create(32*1024, 512*1024, NULL);
       pcre2_jit_stack_assign(mcontext, NULL, jit_stack);
       match_data = pcre2_match_data_create(re, 10);
       rc = pcre2_match(re, subject, length, 0, 0, match_data, mcontext);
       /* Process result */

       pcre2_code_free(re);
       pcre2_match_data_free(match_data);
       pcre2_match_context_free(mcontext);
       pcre2_jit_stack_free(jit_stack);


JIT FAST PATH API
       Because the API described above falls back to interpreted matching when
       JIT is not available, it is convenient for programs that are written
       for general use in many environments. However, calling JIT via
       pcre2_match() does have a performance impact. Programs that are written
       for use where JIT is known to be available, and which need the best
       possible performance, can instead use a "fast path" API to call JIT
       matching directly instead of calling pcre2_match() (obviously only for
       patterns that have been successfully processed by pcre2_jit_compile()).

       The fast path function is called pcre2_jit_match(), and it takes
       exactly the same arguments as pcre2_match(). However, the subject
       string must be specified with a length; PCRE2_ZERO_TERMINATED is not
       supported. Unsupported option bits (for example, PCRE2_ANCHORED,
       PCRE2_ENDANCHORED, and PCRE2_COPY_MATCHED_SUBJECT) are ignored, as is
       the PCRE2_NO_JIT option. The return values are also the same as for
       pcre2_match(), plus PCRE2_ERROR_JIT_BADOPTION if a matching mode
       (partial or complete) is requested that was not compiled.

       When you call pcre2_match(), as well as testing for invalid options, a
       number of other sanity checks are performed on the arguments. For
       example, if the subject pointer is NULL but the length is non-zero, an
       immediate error is given. Also, unless PCRE2_NO_UTF_CHECK is set, a UTF
       subject string is tested for validity. In the interests of speed, these
       checks do not happen on the JIT fast path. If invalid UTF data is
       passed when PCRE2_MATCH_INVALID_UTF was not set for pcre2_compile(),
       the result is undefined. The program may crash or loop or give wrong
       results. In the absence of PCRE2_MATCH_INVALID_UTF you should call
       pcre2_jit_match() in UTF mode only if you are sure the subject is
       valid.

       Bypassing the sanity checks and the pcre2_match() wrapping can give
       speedups of more than 10%.


SEE ALSO
       pcre2api(3), pcre2unicode(3)


AUTHOR
         Philip Hazel (FAQ by Zoltan Herczeg)
         Retired from University Computing Service
         Cambridge, England.


REVISION
       Last updated: 08 August 2026
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.49-DEV                 08 August 2026                     PCRE2JIT(3)
------------------------------------------------------------------------------


PCRE2LIMITS(3)                       PCRE2                      PCRE2LIMITS(3)


NAME
       pcre2limits - PCRE2 resource limits


SIZE AND OTHER LIMITATIONS
       There are some size limitations in PCRE2 but it is hoped that they will
       never in practice be relevant.

       The maximum size of a compiled pattern is approximately 64 thousand
       code units for the 8-bit and 16-bit libraries if PCRE2 is compiled with
       the default internal linkage size, which is 2 bytes for these
       libraries. If you want to process regular expressions that are truly
       enormous, you can compile PCRE2 with an internal linkage size of 3 or 4
       (when building the 16-bit library, 3 is rounded up to 4). See the
       README file in the source distribution and the pcre2build documentation
       for details. In these cases the limit is substantially larger. However,
       the speed of execution is slower. In the 32-bit library, the internal
       linkage size is always 4.

       The maximum length of a source pattern string is essentially unlimited;
       it is the largest number a PCRE2_SIZE variable can hold. However, the
       program that calls pcre2_compile() can specify a smaller limit.

       The maximum length (in code units) of a subject string is one less than
       the largest number a PCRE2_SIZE variable can hold. PCRE2_SIZE is an
       unsigned integer type, usually defined as size_t. Its maximum value
       (that is ~(PCRE2_SIZE)0) is reserved as a special indicator for
       zero-terminated strings and unset offsets.

       All values in repeating quantifiers must be less than 65536.

       There are two different limits that apply to branches of lookbehind
       assertions. If every branch in such an assertion matches a fixed number
       of characters, the maximum length of any branch is 65535 characters. If
       any branch matches a variable number of characters, then the maximum
       matching length for every branch is limited. The default limit is set
       at compile time, defaulting to 255, but can be changed by the calling
       program.

       There is no limit to the number of parenthesized groups, but there can
       be no more than 65535 capture groups, and there is a limit to the depth
       of nesting of parenthesized subpatterns of all kinds. This is imposed
       in order to limit the amount of system stack used at compile time. The
       default limit can be specified when PCRE2 is built; if not, the default
       is set to 250. An application can change this limit by calling
       pcre2_set_parens_nest_limit() to set the limit in a compile context.

       The maximum length of the name for a named capture group as well as the
       number of such groups is configurable at build time. The maximum length
       for the name defaults to 128 code units, and the maximum number of such
       groups to 10000.

       The maximum length of a name in a (*MARK), (*PRUNE), (*SKIP), or
       (*THEN) verb is 255 code units for the 8-bit library and 65535 code
       units for the 16-bit and 32-bit libraries.

       The maximum length of a string argument to a callout is the largest
       number a 32-bit unsigned integer can hold.

       The maximum amount of heap memory used for matching is controlled by
       the heap limit, which can be set in a pattern or in a match context.
       The default is a very large number, effectively unlimited.


AUTHOR
         Philip Hazel
         Retired from University Computing Service
         Cambridge, England.


REVISION
       Last updated: 03 September 2025
       Copyright (c) 1997-2023 University of Cambridge.


PCRE2 10.49-DEV                03 September 2025                PCRE2LIMITS(3)
------------------------------------------------------------------------------


PCRE2MATCHING(3)                     PCRE2                    PCRE2MATCHING(3)


NAME
       pcre2matching - PCRE2 matching algorithms


PCRE2 MATCHING ALGORITHMS
       This document describes the two different algorithms that are available
       in PCRE2 for matching a compiled regular expression against a given
       subject string. The "standard" algorithm is the one provided by the
       pcre2_match() function. This works in the same way as Perl's matching
       function, and provides a Perl-compatible matching operation. The
       just-in-time (JIT) optimization that is described in the pcre2jit
       documentation is compatible with this function.

       An alternative algorithm is provided by the pcre2_dfa_match() function;
       it operates in a different way, and is not Perl-compatible. This
       alternative has advantages and disadvantages compared with the standard
       algorithm, and these are described below.

       When there is only one possible way in which a given subject string can
       match a pattern, the two algorithms give the same answer. A difference
       arises, however, when there are multiple possibilities. For example, if
       the anchored pattern

       ^<.*>

       is matched against the string

       <something> <something else> <something further>

       there are three possible answers. The standard algorithm finds only one
       of them, whereas the alternative algorithm finds all three.


REGULAR EXPRESSIONS AS TREES
       The set of strings that are matched by a regular expression can be
       represented as a tree structure. An unlimited repetition in the pattern
       makes the tree of infinite size, but it is still a tree. Matching the
       pattern to a given subject string (from a given starting point) can be
       thought of as a search of the tree. There are two ways to search a
       tree: depth-first and breadth-first, and these correspond to the two
       matching algorithms provided by PCRE2.


THE STANDARD MATCHING ALGORITHM
       In the terminology of Jeffrey Friedl's book "Mastering Regular
       Expressions", the standard algorithm is an "NFA algorithm". It conducts
       a depth-first search of the pattern tree. That is, it proceeds along a
       single path through the tree, checking that the subject matches what is
       required. When there is a mismatch, the algorithm tries any
       alternatives at the current point, and if they all fail, it backs up to
       the previous branch point in the tree, and tries the next alternative
       branch at that level. This often involves backing up (moving to the
       left) in the subject string as well. The order in which repetition
       branches are tried is controlled by the greedy or ungreedy nature of
       the quantifier.

       If a leaf node is reached, a matching string has been found, and at
       that point the algorithm stops. Thus, if there is more than one
       possible match, this algorithm returns the first one that it finds.
       Whether this is the shortest, the longest, or some intermediate length
       depends on the way the alternations and the greedy or ungreedy
       repetition quantifiers are specified in the pattern.

       Because it ends up with a single path through the tree, it is
       relatively straightforward for this algorithm to keep track of the
       substrings that are matched by portions of the pattern in parentheses.
       This provides support for capturing parentheses and backreferences.


THE ALTERNATIVE MATCHING ALGORITHM
       This algorithm conducts a breadth-first search of the tree. Starting
       from the first matching point in the subject, it scans the subject
       string from left to right, once, character by character, and as it does
       this, it remembers all the paths through the tree that represent valid
       matches. In Friedl's terminology, this is a kind of "DFA algorithm",
       though it is not implemented as a traditional finite state machine (it
       keeps multiple states active simultaneously).

       Although the general principle of this matching algorithm is that it
       scans the subject string only once, without backtracking, there is one
       exception: when a lookaround assertion is encountered, the characters
       following or preceding the current point have to be independently
       inspected.

       The scan continues until either the end of the subject is reached, or
       there are no more unterminated paths. At this point, terminated paths
       represent the different matching possibilities (if there are none, the
       match has failed). Thus, if there is more than one possible match, this
       algorithm finds all of them, and in particular, it finds the longest.
       The matches are returned in the output vector in decreasing order of
       length. There is an option to stop the algorithm after the first match
       (which is necessarily the shortest) is found.

       Note that the size of vector needed to contain all the results depends
       on the number of simultaneous matches, not on the number of capturing
       parentheses in the pattern. Using
       pcre2_match_data_create_from_pattern() to create the match data block
       is therefore not advisable when doing DFA matching.

       Note also that all the matches that are found start at the same point
       in the subject. If the pattern

       cat(er(pillar)?)?

       is matched against the string "the caterpillar catchment", the result
       is the three strings "caterpillar", "cater", and "cat" that start at
       the fifth character of the subject. The algorithm does not
       automatically move on to find matches that start at later positions.

       PCRE2's "auto-possessification" optimization usually applies to
       character repeats at the end of a pattern (as well as internally). For
       example, the pattern a\d+ is compiled as if it were a\d++ because there
       is no point even considering the possibility of backtracking into the
       repeated digits. For DFA matching, this means that only one possible
       match is found. If you really do want multiple matches in such cases,
       either use an ungreedy repeat (a\d+?) or set the PCRE2_NO_AUTO_POSSESS
       option when compiling.

       There are a number of features of PCRE2 regular expressions that are
       not supported or behave differently in the alternative matching
       function. Those that are not supported cause an error if encountered.

        1. Because the algorithm finds all possible matches, the greedy or
           ungreedy nature of repetition quantifiers is not relevant (though
           it may affect auto-possessification, as just described). During
           matching, greedy and ungreedy quantifiers are treated in exactly
           the same way. However, possessive quantifiers can make a difference
           when what follows could also match what is quantified, for example
           in a pattern like this:

       ^a++\w!

       This pattern matches "aaab!" but not "aaa!", which would be matched by
       a non-possessive quantifier. Similarly, if an atomic group is present,
       it is matched as if it were a standalone pattern at the current point,
       and the longest match is then "locked in" for the rest of the overall
       pattern.

        1. When dealing with multiple paths through the tree simultaneously,
           it is not straightforward to keep track of captured substrings for
           the different matching possibilities, and PCRE2's implementation of
           this algorithm does not attempt to do this. This means that no
           captured substrings are available.

        2. Because no substrings are captured, a number of related features
           are not available:

       (a) Backreferences;

       (b) Conditional expressions that use a backreference as the condition
       or test for a specific group recursion;

       (c) Script runs;

       (d) Scan substring assertions.

        1. Because many paths through the tree may be active, the \K escape
           sequence, which resets the start of the match when encountered (but
           may be on some paths and not on others), is not supported.

        2. Callouts are supported, but the value of the capture_top field is
           always 1, and the value of the capture_last field is always 0.

        3. The \C escape sequence, which (in the standard algorithm) always
           matches a single code unit, even in a UTF mode, is not supported in
           UTF modes because the alternative algorithm moves through the
           subject string one character (not code unit) at a time, for all
           active paths through the tree.

        4. Except for (*FAIL), the backtracking control verbs such as (*PRUNE)
           are not supported. (*FAIL) is supported, and behaves like a failing
           negative assertion.

        5. The PCRE2_MATCH_INVALID_UTF option for pcre2_compile() is not
           supported by pcre2_dfa_match().


ADVANTAGES OF THE ALTERNATIVE ALGORITHM
       The main advantage of the alternative algorithm is that all possible
       matches (at a single point in the subject) are automatically found, and
       in particular, the longest match is found. To find more than one match
       at the same point using the standard algorithm, you have to do kludgy
       things with callouts.

       Partial matching is possible with this algorithm, though it has some
       limitations. The pcre2partial documentation gives details of partial
       matching and discusses multi-segment matching.


DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
       The alternative algorithm suffers from a number of disadvantages:

        1. It is substantially slower than the standard algorithm. This is
           partly because it has to search for all possible matches, but is
           also because it is less susceptible to optimization.

        2. Capturing parentheses and other features such as backreferences
           that rely on them are not supported.

        3. Matching within invalid UTF strings is not supported.

        4. Although atomic groups are supported, their use does not provide
           the performance advantage that it does for the standard algorithm.

        5. JIT optimization is not supported.


AUTHOR
         Philip Hazel
         Retired from University Computing Service
         Cambridge, England.


REVISION
       Last updated: 22 February 2025
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.49-DEV                22 February 2025               PCRE2MATCHING(3)
------------------------------------------------------------------------------


PCRE2PARTIAL(3)                      PCRE2                     PCRE2PARTIAL(3)


NAME
       pcre2partial - PCRE2 partial matching


PARTIAL MATCHING IN PCRE2
       In normal use of PCRE2, if there is a match up to the end of a subject
       string, but more characters are needed to match the entire pattern,
       PCRE2_ERROR_NOMATCH is returned, just like any other failing match.
       There are circumstances where it might be helpful to distinguish this
       "partial match" case.

       One example is an application where the subject string is very long,
       and not all available at once. The requirement here is to be able to do
       the matching segment by segment, but special action is needed when a
       matched substring spans the boundary between two segments.

       Another example is checking a user input string as it is typed, to
       ensure that it conforms to a required format. Invalid characters can be
       immediately diagnosed and rejected, giving instant feedback.

       Partial matching is a PCRE2-specific feature; it is not
       Perl-compatible. It is requested by setting one of the
       PCRE2_PARTIAL_HARD or PCRE2_PARTIAL_SOFT options when calling a
       matching function. The difference between the two options is whether or
       not a partial match is preferred to an alternative complete match,
       though the details differ between the two types of matching function.
       If both options are set, PCRE2_PARTIAL_HARD takes precedence.

       If you want to use partial matching with just-in-time optimized code,
       as well as setting a partial match option for the matching function,
       you must also call pcre2_jit_compile() with one or both of these
       options:

       PCRE2_JIT_PARTIAL_HARD
       PCRE2_JIT_PARTIAL_SOFT

       PCRE2_JIT_COMPLETE should also be set if you are going to run
       non-partial matches on the same pattern. Separate code is compiled for
       each mode. If the appropriate JIT mode has not been compiled,
       interpretive matching code is used.

       Setting a partial matching option disables two of PCRE2's standard
       optimization hints. PCRE2 remembers the last literal code unit in a
       pattern, and abandons matching immediately if it is not present in the
       subject string. This optimization cannot be used for a subject string
       that might match only partially. PCRE2 also remembers a minimum length
       of a matching string, and does not bother to run the matching function
       on shorter strings. This optimization is also disabled for partial
       matching.


REQUIREMENTS FOR A PARTIAL MATCH
       A possible partial match occurs during matching when the end of the
       subject string is reached successfully, but either more characters are
       needed to complete the match, or the addition of more characters might
       change what is matched.

       Example 1: if the pattern is abc and the subject is "ab", more
       characters are definitely needed to complete a match. In this case both
       hard and soft matching options yield a partial match.

       Example 2: if the pattern is ab+ and the subject is "ab", a complete
       match can be found, but the addition of more characters might change
       what is matched. In this case, only PCRE2_PARTIAL_HARD returns a
       partial match; PCRE2_PARTIAL_SOFT returns the complete match.

       On reaching the end of the subject, when PCRE2_PARTIAL_HARD is set, if
       the next pattern item is \z, \Z, \b, \B, or $ there is always a partial
       match. Otherwise, for both options, the next pattern item must be one
       that inspects a character, and at least one of the following must be
       true:

       (1) At least one character has already been inspected. An inspected
       character need not form part of the final matched string; lookbehind
       assertions and the \K escape sequence provide ways of inspecting
       characters before the start of a matched string.

       (2) The pattern contains one or more lookbehind assertions. This
       condition exists in case there is a lookbehind that inspects characters
       before the start of the match.

       (3) There is a special case when the whole pattern can match an empty
       string. When the starting point is at the end of the subject, the empty
       string match is a possibility, and if PCRE2_PARTIAL_SOFT is set and
       neither of the above conditions is true, it is returned. However,
       because adding more characters might result in a non-empty match,
       PCRE2_PARTIAL_HARD returns a partial match, which in this case means
       "there is going to be a match at this point, but until some more
       characters are added, we do not know if it will be an empty string or
       something longer".


PARTIAL MATCHING USING pcre2_match()
       When a partial matching option is set, the result of calling
       pcre2_match() can be one of the following:

           A successful match  A complete match has been found, starting and
                               ending within this subject.
           PCRE2_ERROR_NOMATCH No match can start anywhere in this subject.
           PCRE2_ERROR_PARTIAL Adding more characters may result in a complete
                               match that uses one or more characters from the
                               end of this subject.

       When a partial match is returned, the first two elements in the ovector
       point to the portion of the subject that was matched, but the values in
       the rest of the ovector are undefined. The appearance of \K in the
       pattern has no effect for a partial match. Consider this pattern:

       /abc\K123/

       If it is matched against "456abc123xyz" the result is a complete match,
       and the ovector defines the matched string as "123", because \K resets
       the "start of match" point. However, if a partial match is requested
       and the subject string is "456abc12", a partial match is found for the
       string "abc12", because all these characters are needed for a
       subsequent re-match with additional characters.

       If there is more than one partial match, the first one that was found
       provides the data that is returned. Consider this pattern:

       /123\w+X|dogY/

       If this is matched against the subject string "abc123dog", both
       alternatives fail to match, but the end of the subject is reached
       during matching, so PCRE2_ERROR_PARTIAL is returned. The offsets are
       set to 3 and 9, identifying "123dog" as the first partial match. (In
       this example, there are two partial matches, because "dog" on its own
       partially matches the second alternative.)

   How a partial match is processed by pcre2_match()
       What happens when a partial match is identified depends on which of the
       two partial matching options is set.

       If PCRE2_PARTIAL_HARD is set, PCRE2_ERROR_PARTIAL is returned as soon
       as a partial match is found, without continuing to search for possible
       complete matches. This option is "hard" because it prefers an earlier
       partial match over a later complete match. For this reason, the
       assumption is made that the end of the supplied subject string is not
       the true end of the available data, which is why \z, \Z, \b, \B, and $
       always give a partial match.

       If PCRE2_PARTIAL_SOFT is set, the partial match is remembered, but
       matching continues as normal, and other alternatives in the pattern are
       tried. If no complete match can be found, PCRE2_ERROR_PARTIAL is
       returned instead of PCRE2_ERROR_NOMATCH. This option is "soft" because
       it prefers a complete match over a partial match. All the various
       matching items in a pattern behave as if the subject string is
       potentially complete; \z, \Z, and $ match at the end of the subject, as
       normal, and for \b and \B the end of the subject is treated as a
       non-alphanumeric.

       The difference between the two partial matching options can be
       illustrated by a pattern such as:

       /dog(sbody)?/

       This matches either "dog" or "dogsbody", greedily (that is, it prefers
       the longer string if possible). If it is matched against the string
       "dog" with PCRE2_PARTIAL_SOFT, it yields a complete match for "dog".
       However, if PCRE2_PARTIAL_HARD is set, the result is
       PCRE2_ERROR_PARTIAL. On the other hand, if the pattern is made ungreedy
       the result is different:

       /dog(sbody)??/

       In this case the result is always a complete match because that is
       found first, and matching never continues after finding a complete
       match. It might be easier to follow this explanation by thinking of the
       two patterns like this:

       /dog(sbody)?/    is the same as  /dogsbody|dog/
       /dog(sbody)??/   is the same as  /dog|dogsbody/

       The second pattern will never match "dogsbody", because it will always
       find the shorter match first.

   Example of partial matching using pcre2test
       The pcre2test data modifiers partial_hard (or ph) and partial_soft (or
       ps) set PCRE2_PARTIAL_HARD and PCRE2_PARTIAL_SOFT, respectively, when
       calling pcre2_match(). Here is a run of pcre2test using a pattern that
       matches the whole subject in the form of a date:

         re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
       data> 25dec3\=ph
       Partial match: 23dec3
       data> 3ju\=ph
       Partial match: 3ju
       data> 3juj\=ph
       No match

       This example gives the same results for both hard and soft partial
       matching options. Here is an example where there is a difference:

         re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
       data> 25jun04\=ps
        0: 25jun04
        1: jun
       data> 25jun04\=ph
       Partial match: 25jun04

       With PCRE2_PARTIAL_SOFT, the subject is matched completely. For
       PCRE2_PARTIAL_HARD, however, the subject is assumed not to be complete,
       so there is only a partial match.


MULTI-SEGMENT MATCHING WITH pcre2_match()
       PCRE was not originally designed with multi-segment matching in mind.
       However, over time, features (including partial matching) that make
       multi-segment matching possible have been added. A very long string can
       be searched segment by segment by calling pcre2_match() repeatedly,
       with the aim of achieving the same results that would happen if the
       entire string was available for searching all the time. Normally, the
       strings that are being sought are much shorter than each individual
       segment, and are in the middle of very long strings, so the pattern is
       normally not anchored.

       Special logic must be implemented to handle a matched substring that
       spans a segment boundary. PCRE2_PARTIAL_HARD should be used, because it
       returns a partial match at the end of a segment whenever there is the
       possibility of changing the match by adding more characters. The
       PCRE2_NOTBOL option should also be set for all but the first segment.

       When a partial match occurs, the next segment must be added to the
       current subject and the match re-run, using the startoffset argument of
       pcre2_match() to begin at the point where the partial match started.
       For example:

         re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
       data> ...the date is 23ja\=ph
       Partial match: 23ja
       data> ...the date is 23jan19 and on that day...\=offset=15
        0: 23jan19
        1: jan

       Note the use of the offset modifier to start the new match where the
       partial match was found. In this example, the next segment was added to
       the one in which the partial match was found. This is the most
       straightforward approach, typically using a memory buffer that is twice
       the size of each segment. After a partial match, the first half of the
       buffer is discarded, the second half is moved to the start of the
       buffer, and a new segment is added before repeating the match as in the
       example above. After a no match, the entire buffer can be discarded.

       If there are memory constraints, you may want to discard text that
       precedes a partial match before adding the next segment. Unfortunately,
       this is not at present straightforward. In cases such as the above,
       where the pattern does not contain any lookbehinds, it is sufficient to
       retain only the partially matched substring. However, if the pattern
       contains a lookbehind assertion, characters that precede the start of
       the partial match may have been inspected during the matching process.
       When pcre2test displays a partial match, it indicates these characters
       with '<' if the allusedtext modifier is set:

         re> "(?<=123)abc"
       data> xx123ab\=ph,allusedtext
       Partial match: 123ab
                      <<<

       However, the allusedtext modifier is not available for JIT matching,
       because JIT matching does not record the first (or last) consulted
       characters. For this reason, this information is not available via the
       API. It is therefore not possible in general to obtain the exact number
       of characters that must be retained in order to get the right match
       result. If you cannot retain the entire segment, you must find some
       heuristic way of choosing.

       If you know the approximate length of the matching substrings, you can
       use that to decide how much text to retain. The only lookbehind
       information that is currently available via the API is the length of
       the longest individual lookbehind in a pattern, but this can be
       misleading if there are nested lookbehinds. The value returned by
       calling pcre2_pattern_info() with the PCRE2_INFO_MAXLOOKBEHIND option
       is the maximum number of characters (not code units) that any
       individual lookbehind moves back when it is processed. A pattern such
       as (?<=(?<!b)a) has a maximum lookbehind value of one, but inspects two
       characters before its starting point.

       In a non-UTF or a 32-bit case, moving back is just a subtraction, but
       in UTF-8 or UTF-16 you have to count characters while moving back
       through the code units.


PARTIAL MATCHING USING pcre2_dfa_match()
       The DFA function moves along the subject string character by character,
       without backtracking, searching for all possible matches
       simultaneously. If the end of the subject is reached before the end of
       the pattern, there is the possibility of a partial match.

       When PCRE2_PARTIAL_SOFT is set, PCRE2_ERROR_PARTIAL is returned only if
       there have been no complete matches. Otherwise, the complete matches
       are returned. If PCRE2_PARTIAL_HARD is set, a partial match takes
       precedence over any complete matches. The portion of the string that
       was matched when the longest partial match was found is set as the
       first matching string.

       Because the DFA function always searches for all possible matches, and
       there is no difference between greedy and ungreedy repetition, its
       behaviour is different from pcre2_match(). Consider the string "dog"
       matched against this ungreedy pattern:

       /dog(sbody)??/

       Whereas the standard function stops as soon as it finds the complete
       match for "dog", the DFA function also finds the partial match for
       "dogsbody", and so returns that when PCRE2_PARTIAL_HARD is set.


MULTI-SEGMENT MATCHING WITH pcre2_dfa_match()
       When a partial match has been found using the DFA matching function, it
       is possible to continue the match by providing additional subject data
       and calling the function again with the same compiled regular
       expression, this time setting the PCRE2_DFA_RESTART option. You must
       pass the same working space as before, because this is where details of
       the previous partial match are stored. You can set the
       PCRE2_PARTIAL_SOFT or PCRE2_PARTIAL_HARD options with PCRE2_DFA_RESTART
       to continue partial matching over multiple segments. Here is an example
       using pcre2test:

         re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
       data> 23ja\=dfa,ps
       Partial match: 23ja
       data> n05\=dfa,dfa_restart
        0: n05

       The first call has "23ja" as the subject, and requests partial
       matching; the second call has "n05" as the subject for the continued
       (restarted) match. Notice that when the match is complete, only the
       last part is shown; PCRE2 does not retain the previously
       partially-matched string. It is up to the calling program to do that if
       it needs to. This means that, for an unanchored pattern, if a continued
       match fails, it is not possible to try again at a new starting point.
       All this facility is capable of doing is continuing with the previous
       match attempt. For example, consider this pattern:

       1234|3789

       If the first part of the subject is "ABC123", a partial match of the
       first alternative is found at offset 3. There is no partial match for
       the second alternative, because such a match does not start at the same
       point in the subject string. Attempting to continue with the string
       "7890" does not yield a match because only those alternatives that
       match at one point in the subject are remembered. Depending on the
       application, this may or may not be what you want.

       If you do want to allow for starting again at the next character, one
       way of doing it is to retain some or all of the segment and try a new
       complete match, as described for pcre2_match() above. Another
       possibility is to work with two buffers. If a partial match at offset n
       in the first buffer is followed by "no match" when PCRE2_DFA_RESTART is
       used on the second buffer, you can then try a new match starting at
       offset n+1 in the first buffer.

       Therefore, PCRE2_DFA_RESTART does not remove the need to keep earlier
       subject data. In order to perform a bumpalong and continue matching at
       the next character, all subject data from the current match start must
       be retained, together with enough preceding data to satisfy the
       requirements of lookbehind assertions (remembering as above that
       PCRE2_INFO_MAXLOOKBEHIND only describes the buffer requirement of a
       single lookbehind, and nested lookbehinds can read back further from
       the match start). This makes streaming match processing using
       PCRE2_DFA_RESTART challenging, and the application cannot easily set a
       bound on the number of chunks to buffer. For multi-segment searching
       that is intended to produce the same result as searching the whole
       subject, it is usually simpler to avoid PCRE2_DFA_RESTART and run a
       fresh match, using either matching function, over the retained text.


AUTHOR
         Philip Hazel
         Retired from University Computing Service
         Cambridge, England.


REVISION
       Last updated: 16 August 2026
       Copyright (c) 1997-2019 University of Cambridge.


PCRE2 10.49-DEV                 16 August 2026                 PCRE2PARTIAL(3)
------------------------------------------------------------------------------


PCRE2PATTERN(3)                      PCRE2                     PCRE2PATTERN(3)


NAME
       pcre2pattern - PCRE2 pattern syntax


PCRE2 REGULAR EXPRESSION DETAILS
       The syntax and semantics of the regular expressions that are supported
       by PCRE2 are described in detail below. There is a quick-reference
       syntax summary in the pcre2syntax page. PCRE2 tries to match Perl
       syntax and semantics as closely as it can. PCRE2 also supports some
       alternative regular expression syntax that does not conflict with the
       Perl syntax in order to provide some compatibility with regular
       expressions in Python, .NET, and Oniguruma. There are in addition some
       options that enable alternative syntax and semantics that are not the
       same as in Perl.

       Perl's regular expressions are described in its own documentation, and
       regular expressions in general are covered in a number of books, some
       of which have copious examples. Jeffrey Friedl's "Mastering Regular
       Expressions", published by O'Reilly, covers regular expressions in
       great detail. This description of PCRE2's regular expressions is
       intended as reference material.

       This document discusses the regular expression patterns that are
       supported by PCRE2 when its main matching function, pcre2_match(), is
       used. PCRE2 also has an alternative matching function,
       pcre2_dfa_match(), which matches using a different algorithm that is
       not Perl-compatible. Some of the features discussed below are not
       available when DFA matching is used. The advantages and disadvantages
       of the alternative function, and how it differs from the normal
       function, are discussed in the pcre2matching page.


EBCDIC CHARACTER CODES
       Most computers use ASCII or Unicode for encoding characters, and PCRE2
       assumes this by default. However, it can be compiled to run in an
       environment that uses the EBCDIC code, which is the case for some IBM
       mainframe operating systems. In the sections below, character code
       values are ASCII or Unicode; in an EBCDIC environment these characters
       may have different code values, and there are no code points greater
       than 255. Differences in behaviour when PCRE2 is running in an EBCDIC
       environment are described in the section "EBCDIC environments" below,
       which you can ignore unless you really are in an EBCDIC environment.


SPECIAL START-OF-PATTERN ITEMS
       A number of options that can be passed to pcre2_compile() can also be
       set by special items at the start of a pattern. These are not
       Perl-compatible, but are provided to make these options accessible to
       pattern writers who are not able to change the program that processes
       the pattern. Any number of these items may appear, but they must all be
       together right at the start of the pattern string, and the letters must
       be in upper case.

   UTF support
       In the 8-bit and 16-bit PCRE2 libraries, characters may be coded either
       as single code units, or as multiple UTF-8 or UTF-16 code units. UTF-32
       can be specified for the 32-bit library, in which case it constrains
       the character values to valid Unicode code points. To process UTF
       strings, PCRE2 must be built to include Unicode support (which is the
       default). When using UTF strings you must either call the compiling
       function with one or both of the PCRE2_UTF or PCRE2_MATCH_INVALID_UTF
       options, or the pattern must start with the special sequence (*UTF),
       which is equivalent to setting the relevant PCRE2_UTF. How setting a
       UTF mode affects pattern matching is mentioned in several places below.
       There is also a summary of features in the pcre2unicode page.

       Some applications that allow their users to supply patterns may wish to
       restrict them to non-UTF data for security reasons. If the
       PCRE2_NEVER_UTF option is passed to pcre2_compile(), (*UTF) is not
       allowed, and its appearance in a pattern causes an error.

   Unicode property support
       Another special sequence that may appear at the start of a pattern is
       (*UCP). This has the same effect as setting the PCRE2_UCP option: it
       causes sequences such as \d and \w to use Unicode properties to
       determine character types, instead of recognizing only characters with
       codes less than 256 via a lookup table. If also causes upper/lower
       casing operations to use Unicode properties for characters with code
       points greater than 127, even when UTF is not set. These behaviours can
       be changed within the pattern; see the section entitled "Internal
       Option Setting" below.

       Some applications that allow their users to supply patterns may wish to
       restrict them for security reasons. If the PCRE2_NEVER_UCP option is
       passed to pcre2_compile(), (*UCP) is not allowed, and its appearance in
       a pattern causes an error.

   Locking out empty string matching
       Starting a pattern with (*NOTEMPTY) or (*NOTEMPTY_ATSTART) has the same
       effect as passing the PCRE2_NOTEMPTY or PCRE2_NOTEMPTY_ATSTART option
       to whichever matching function is subsequently called to match the
       pattern. These options lock out the matching of empty strings, either
       entirely, or only at the start of the subject.

   Disabling auto-possessification
       If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as
       setting the PCRE2_NO_AUTO_POSSESS option, or calling
       pcre2_set_optimize() with a PCRE2_AUTO_POSSESS_OFF directive. This
       stops PCRE2 from making quantifiers possessive when what follows cannot
       match the repeated item. For example, by default a+b is treated as
       a++b. For more details, see the pcre2api documentation.

   Disabling start-up optimizations
       If a pattern starts with (*NO_START_OPT), it has the same effect as
       setting the PCRE2_NO_START_OPTIMIZE option, or calling
       pcre2_set_optimize() with a PCRE2_START_OPTIMIZE_OFF directive. This
       disables several optimizations for quickly reaching "no match" results.
       For more details, see the pcre2api documentation.

   Disabling automatic anchoring
       If a pattern starts with (NO_DOTSTAR_ANCHOR), it has the same effect as
       setting the PCRE2_NO_DOTSTAR_ANCHOR option, or calling
       pcre2_set_optimize() with a PCRE2_DOTSTAR_ANCHOR_OFF directive. This
       disables optimizations that apply to patterns whose top-level branches
       all start with . (match any number of arbitrary characters). For more
       details, see the pcre2api documentation.

   Disabling JIT compilation
       If a pattern that starts with (*NO_JIT) is successfully compiled, an
       attempt by the application to apply the JIT optimization by calling
       pcre2_jit_compile() is ignored.

   Setting match resource limits
       The pcre2_match() function contains a counter that is incremented every
       time it goes round its main loop. The caller of pcre2_match() can set a
       limit on this counter, which therefore limits the amount of computing
       resource used for a match. The maximum depth of nested backtracking can
       also be limited; this indirectly restricts the amount of heap memory
       that is used, but there is also an explicit memory limit that can be
       set.

       These facilities are provided to catch runaway matches that are
       provoked by patterns with huge matching trees. A common example is a
       pattern with nested unlimited repeats applied to a long string that
       does not match. When one of these limits is reached, pcre2_match()
       gives an error return. The limits can also be set by items at the start
       of the pattern of the form

       (*LIMIT_HEAP=d)
       (*LIMIT_MATCH=d)
       (*LIMIT_DEPTH=d)

       where d is any number of decimal digits. However, the value of the
       setting must be less than the value set (or defaulted) by the caller of
       pcre2_match() for it to have any effect. In other words, the pattern
       writer can lower the limits set by the programmer, but not raise them.
       If there is more than one setting of one of these limits, the lower
       value is used. The heap limit is specified in kibibytes (units of 1024
       bytes).

       Prior to release 10.30, LIMIT_DEPTH was called LIMIT_RECURSION. This
       name is still recognized for backwards compatibility.

       The heap limit applies only when the pcre2_match() or pcre2_dfa_match()
       interpreters are used for matching. It does not apply to JIT. The match
       limit is used (but in a different way) when JIT is being used, or when
       pcre2_dfa_match() is called, to limit computing resource usage by those
       matching functions. The depth limit is ignored by JIT but is relevant
       for DFA matching, which uses function recursion for recursions within
       the pattern and for lookaround assertions and atomic groups. In this
       case, the depth limit controls the depth of such recursion.

   Newline conventions
       PCRE2 supports six different conventions for indicating line breaks in
       strings: a single CR (carriage return) character, a single LF
       (linefeed) character, the two-character sequence CRLF, any of the three
       preceding, any Unicode newline sequence, or the NUL character (binary
       zero). The pcre2api page has further discussion about newlines, and
       shows how to set the newline convention when calling pcre2_compile().

       It is also possible to specify a newline convention by starting a
       pattern string with one of the following sequences:

       (*CR)        carriage return
       (*LF)        linefeed
       (*CRLF)      carriage return, followed by linefeed
       (*ANYCRLF)   any of the three above
       (*ANY)       all Unicode newline sequences
       (*NUL)       the NUL character (binary zero)

       These override the default and the options given to the compiling
       function. For example, on a Unix system where LF is the default newline
       sequence, the pattern

       (*CR)a.b

       changes the convention to CR. That pattern matches "a\nb" because LF is
       no longer a newline. If more than one of these settings is present, the
       last one is used.

       The newline convention affects where the circumflex and dollar
       assertions are true. It also affects the interpretation of the dot
       metacharacter when PCRE2_DOTALL is not set, and the behaviour of \N
       when not followed by an opening brace. However, it does not affect what
       the \R escape sequence matches. By default, this is any Unicode newline
       sequence, for Perl compatibility. However, this can be changed; see the
       next section and the description of \R in the section entitled "Newline
       sequences" below. A change of \R setting can be combined with a change
       of newline convention.

   Specifying what \R matches
       It is possible to restrict \R to match only CR, LF, or CRLF (instead of
       the complete set of Unicode line endings) by setting the option
       PCRE2_BSR_ANYCRLF at compile time. This effect can also be achieved by
       starting a pattern with (*BSR_ANYCRLF). For completeness,
       (*BSR_UNICODE) is also recognized, corresponding to PCRE2_BSR_UNICODE.


CHARACTERS AND METACHARACTERS
       A regular expression is a pattern that is matched against a subject
       string from left to right. Most characters stand for themselves in a
       pattern, and match the corresponding characters in the subject. As a
       trivial example, the pattern

       The quick brown fox

       matches a portion of a subject string that is identical to itself. When
       caseless matching is specified (the PCRE2_CASELESS option or (?i)
       within the pattern), letters are matched independently of case. Note
       that there are two ASCII characters, K and S, that, in addition to
       their lower case ASCII equivalents, are case-equivalent with Unicode
       U+212A (Kelvin sign) and U+017F (long S) respectively when either
       PCRE2_UTF or PCRE2_UCP is set, unless the PCRE2_EXTRA_CASELESS_RESTRICT
       option is in force (either passed to pcre2_compile() or set by
       (*CASELESS_RESTRICT) or (?r) within the pattern). If the
       PCRE2_EXTRA_TURKISH_CASING option is in force (either passed to
       pcre2_compile() or set by (*TURKISH_CASING) within the pattern), then
       the 'i' letters are matched according to Turkish and Azeri languages.

       The power of regular expressions comes from the ability to include wild
       cards, character classes, alternatives, and repetitions in the pattern.
       These are encoded in the pattern by the use of metacharacters, which do
       not stand for themselves but instead are interpreted in some special
       way.

       There are two different sets of metacharacters: those that are
       recognized anywhere in the pattern except within square brackets, and
       those that are recognized within square brackets. Outside square
       brackets, the metacharacters are as follows:

       \      general escape character with several uses
       ^      assert start of string (or line, in multiline mode)
       $      assert end of string (or line, in multiline mode)
       .      match any character except newline (by default)
       [      start character class definition
       |      start of alternative branch
       (      start group or control verb
       )      end group or control verb
       *      0 or more quantifier
       +      1 or more quantifier; also "possessive quantifier"
       ?      0 or 1 quantifier; also quantifier minimizer
       {      potential start of min/max quantifier

       Brace characters { and } are also used to enclose data for
       constructions such as \g{2} or \k{name}. In almost all uses of braces,
       space and/or horizontal tab characters that follow { or precede } are
       allowed and are ignored. In the case of quantifiers, they may also
       appear before or after the comma. The exception to this is \u{...}
       which is an ECMAScript compatibility feature that is recognized only
       when the PCRE2_EXTRA_ALT_BSUX option is set. ECMAScript does not ignore
       such white space; it causes the item to be interpreted as literal.

       Part of a pattern that is in square brackets is called a "character
       class". In a character class the only metacharacters are:

       \      general escape character
       ^      negate the class, but only if the first character
       -      indicates character range
       [      POSIX character class (if followed by POSIX syntax)
       ]      terminates the character class

       If a pattern is compiled with the PCRE2_EXTENDED option, most white
       space in the pattern, other than in a character class, within a \Q...\E
       sequence, or between a # outside a character class and the next
       newline, inclusive, is ignored. An escaping backslash can be used to
       include a white space or a # character as part of the pattern. If the
       PCRE2_EXTENDED_MORE option is set, the same applies, but in addition
       unescaped space and horizontal tab characters are ignored inside a
       character class. Note: only these two characters are ignored, not the
       full set of pattern white space characters that are ignored outside a
       character class. Option settings can be changed within a pattern; see
       the section entitled "Internal Option Setting" below.

       The following sections describe the use of each of the metacharacters.


BACKSLASH
       The backslash character has several uses. Firstly, if it is followed by
       a character that is not a digit or a letter, it takes away any special
       meaning that character may have. This use of backslash as an escape
       character applies both inside and outside character classes.

       For example, if you want to match a  character, you must write \ in the
       pattern. This escaping action applies whether or not the following
       character would otherwise be interpreted as a metacharacter, so it is
       always safe to precede a non-alphanumeric with backslash to specify
       that it stands for itself. In particular, if you want to match a
       backslash, you write \\.

       Only ASCII digits and letters have any special meaning after a
       backslash. All other characters (in particular, those whose code points
       are greater than 127) are treated as literals.

       If you want to treat all characters in a sequence as literals, you can
       do so by putting them between \Q and \E. Note that this includes white
       space even when the PCRE2_EXTENDED option is set so that most other
       white space is ignored. The behaviour is different from Perl in that $
       and @ are handled as literals in \Q...\E sequences in PCRE2, whereas in
       Perl, $ and @ cause variable interpolation. Also, Perl does
       "double-quotish backslash interpolation" on any backslashes between \Q
       and \E which, its documentation says, "may lead to confusing results".
       PCRE2 treats a backslash between \Q and \E just like any other
       character. Note the following examples:

       Pattern            PCRE2 matches   Perl matches

       \Qabc$xyz\E        abc$xyz        abc followed by the
                                           contents of $xyz
       \Qabc\$xyz\E       abc\$xyz       abc\$xyz
       \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz
       \QA\B\E            A\B            A\B
       \Q\\E              \              \\E

       The \Q...\E sequence is recognized both inside and outside character
       classes. An isolated \E that is not preceded by \Q is ignored. If \Q is
       not followed by \E later in the pattern, the literal interpretation
       continues to the end of the pattern (that is, \E is assumed at the
       end). If the isolated \Q is inside a character class, this causes an
       error, because the character class is then not terminated by a closing
       square bracket.

       Another difference from Perl is that any appearance of \Q or \E inside
       what might otherwise be a quantifier causes PCRE2 not to recognize the
       sequence as a quantifier. Perl recognizes a quantifier if (redundantly)
       either of the numbers is inside \Q...\E, but not if the separating
       comma is. When not recognized as a quantifier a sequence such as
       {\Q1\E,2} is treated as the literal string "{1,2}".

   Non-printing characters
       A second use of backslash provides a way of encoding non-printing
       characters in patterns in a visible manner. There is no restriction on
       the appearance of non-printing characters in a pattern, but when a
       pattern is being prepared by text editing, it is often easier to use
       one of the following escape sequences instead of the binary character
       it represents. In an ASCII or Unicode environment, these escapes are as
       follows:

       \a          alarm, that is, the BEL character (hex 07)
       \cx         "control-x", where x is a non-control ASCII character
       \e          escape (hex 1B)
       \f          form feed (hex 0C)
       \n          linefeed (hex 0A)
       \r          carriage return (hex 0D) (but see below)
       \t          tab (hex 09)
       \0dd        character with octal code 0dd
       \ddd        character with octal code ddd, or back reference
       \o{ddd..}   character with octal code ddd..
       \xhh        character with hex code hh
       \x{hhh..}   character with hex code hhh..
       \N{U+hhh..} character with Unicode hex code point hhh..

       A description of how back references work is given later, following the
       discussion of parenthesized groups.

       By default, after \x that is not followed by {, one or two hexadecimal
       digits are read (letters can be in upper or lower case). If the
       character that follows \x is neither { nor a hexadecimal digit, an
       error occurs. This is different from Perl's default behaviour, which
       generates a NUL character, but is in line with the behaviour of Perl's
       'strict' mode in re.

       Any number of hexadecimal digits may appear between \x{ and }. If a
       character other than a hexadecimal digit appears between \x{ and }, or
       if there is no terminating }, an error occurs.

       Characters whose code points are less than 256 can be defined by either
       of the two syntaxes for \x or by an octal sequence. There is no
       difference in the way they are handled. For example, \xdc is exactly
       the same as \x{dc} or \334. However, using the braced versions does
       make such sequences easier to read.

       Support is available for some ECMAScript (aka JavaScript) escape
       sequences via two compile-time options. If PCRE2_ALT_BSUX is set, the
       sequence \x followed by { is not recognized. Only if \x is followed by
       two hexadecimal digits is it recognized as a character escape.
       Otherwise it is interpreted as a literal "x" character. In this mode,
       support for code points greater than 256 is provided by \u, which must
       be followed by four hexadecimal digits; otherwise it is interpreted as
       a literal "u" character.

       PCRE2_EXTRA_ALT_BSUX has the same effect as PCRE2_ALT_BSUX and, in
       addition, \u{hhh..} is recognized as the character specified by
       hexadecimal code point. There may be any number of hexadecimal digits,
       but unlike other places that also use curly brackets, spaces are not
       allowed and would result in the string being interpreted as a literal.
       This syntax is from ECMAScript 6.

       The \N{U+hhh..} escape sequence is recognized only when PCRE2 is
       operating in UTF mode. Perl also uses \N{name} to specify characters by
       Unicode name; PCRE2 does not support this. Note that when \N is not
       followed by an opening brace (curly bracket) it has an entirely
       different meaning, matching any character that is not a newline.

       There are some legacy applications where the escape sequence \r is
       expected to match a newline. If the PCRE2_EXTRA_ESCAPED_CR_IS_LF option
       is set, \r in a pattern is converted to \n so that it matches a LF
       (linefeed) instead of a CR (carriage return) character.

       An error occurs if \c is not followed by a character whose ASCII code
       point is in the range 32 to 126. The precise effect of \cx is as
       follows: if x is a lower case letter, it is converted to upper case.
       Then bit 6 of the character (hex 40) is inverted. Thus \cA to \cZ
       become hex 01 to hex 1A (A is 41, Z is 5A), but \c{ becomes hex 3B ({
       is 7B), and \c; becomes hex 7B (; is 3B). If the code unit following \c
       has a code point less than 32 or greater than 126, a compile-time error
       occurs.

       For differences in the way some escapes behave in EBCDIC environments,
       see section "EBCDIC environments" below.

   Octal escapes and back references
       The escape \o must be followed by a sequence of octal digits, enclosed
       in braces. An error occurs if this is not the case. This escape
       provides a way of specifying character code points as octal numbers
       greater than 0777, and it also allows octal numbers and backreferences
       to be unambiguously distinguished.

       If braces are not used, after \0 up to two further octal digits are
       read. However, if the PCRE2_EXTRA_NO_BS0 option is set, at least one
       more octal digit must follow \0 (use \00 to generate a NUL character).
       Make sure you supply two digits after the initial zero if the pattern
       character that follows is itself an octal digit.

       Inside a character class, when a backslash is followed by any octal
       digit, up to three octal digits are read to generate a code point. Any
       subsequent digits stand for themselves. The sequences \8 and \9 are
       treated as the literal characters "8" and "9".

       Outside a character class, Perl's handling of a backslash followed by a
       digit other than 0 is complicated by ambiguity, and Perl has changed
       over time, causing PCRE2 also to change. From PCRE2 release 10.45 there
       is an option called PCRE2_EXTRA_PYTHON_OCTAL that causes PCRE2 to use
       Python's unambiguous rules. The next two subsections describe the two
       sets of rules.

       For greater clarity and unambiguity, it is best to avoid following \ by
       a digit greater than zero. Instead, use \o{...} or \x{...} to specify
       numerical character code points, and \g{...} to specify backreferences.

   Perl rules for non-class backslash 1-9
       All the digits that follow the backslash are read as a decimal number.
       If the number is less than 10, begins with the digit 8 or 9, or if
       there are at least that many previous capture groups in the expression,
       the entire sequence is taken as a back reference. Otherwise, up to
       three octal digits are read to form a character code. For example:

       \040   is another way of writing an ASCII space
       \40    is the same, provided there are fewer than 40
                 previous capture groups
       \7     is always a backreference
       \11    might be a backreference, or another way of
                 writing a tab
       \011   is always a tab
       \0113  is a tab followed by the character "3"
       \113   might be a backreference, otherwise the
                 character with octal code 113
       \377   might be a backreference, otherwise
                 the value 255 (decimal)
       \81    is always a backreference

       Note that octal values of 100 or greater that are specified using this
       syntax must not be introduced by a leading zero, because no more than
       three octal digits are ever read.

   Python rules for non-class backslash 1-9
       If there are at least three octal digits after the backslash, exactly
       three are read as an octal code point number, but the value must be no
       greater than \377, even in modes where higher code point values are
       supported. Any subsequent digits stand for themselves. If there are
       fewer than three octal digits, the sequence is taken as a decimal back
       reference. Thus, for example, \12 is always a back reference,
       independent of how many captures there are in the pattern. An error is
       generated for a reference to a non-existent capturing group.

   Constraints on character values
       Characters that are specified using octal or hexadecimal numbers are
       limited to certain values, as follows:

       8-bit non-UTF mode    no greater than 0xff
       16-bit non-UTF mode   no greater than 0xffff
       32-bit non-UTF mode   no greater than 0xffffffff
       All UTF modes         no greater than 0x10ffff and a valid code point

       Invalid Unicode code points are all those in the range 0xd800 to 0xdfff
       (the so-called "surrogate" code points). The check for these can be
       disabled by the caller of pcre2_compile() by setting the option
       PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES. However, this is possible only in
       UTF-8 and UTF-32 modes, because these values are not representable in
       UTF-16.

   Escape sequences in character classes
       All the sequences that define a single character value can be used both
       inside and outside character classes. In addition, inside a character
       class, \b is interpreted as the backspace character (hex 08).

       When not followed by an opening brace, \N is not allowed in a character
       class. \B, \R, and \X are not special inside a character class. Like
       other unrecognized alphabetic escape sequences, they cause an error.
       Outside a character class, these sequences have different meanings.

   Unsupported escape sequences
       In Perl, the sequences \F, \l, \L, \u, and \U are recognized by its
       string handler and used to modify the case of following characters. By
       default, PCRE2 does not support these escape sequences in patterns.
       However, if either of the PCRE2_ALT_BSUX or PCRE2_EXTRA_ALT_BSUX
       options is set, \U matches a "U" character, and \u can be used to
       define a character by code point, as described above.

   Absolute and relative backreferences
       The sequence \g followed by a signed or unsigned number, optionally
       enclosed in braces, is an absolute or relative backreference. A named
       backreference can be coded as \g{name}. Backreferences are discussed
       later, following the discussion of parenthesized groups.

   Absolute and relative subroutine calls
       For compatibility with Oniguruma, the non-Perl syntax \g followed by a
       name or a number enclosed either in angle brackets or single quotes, is
       an alternative syntax for referencing a capture group as a subroutine.
       Details are discussed later. Note that \g{...} (Perl syntax) and
       \g<...> (Oniguruma syntax) are not synonymous. The former is a
       backreference; the latter is a subroutine call.

   Generic character types
       Another use of backslash is for specifying generic character types:

       \d     any decimal digit
       \D     any character that is not a decimal digit
       \h     any horizontal white space character
       \H     any character that is not a horizontal white space character
       \N     any character that is not a newline
       \s     any white space character
       \S     any character that is not a white space character
       \v     any vertical white space character
       \V     any character that is not a vertical white space character
       \w     any "word" character
       \W     any "non-word" character

       The \N escape sequence has the same meaning as the "." metacharacter
       when PCRE2_DOTALL is not set, but setting PCRE2_DOTALL does not change
       the meaning of \N. Note that when \N is followed by an opening brace it
       has a different meaning. See the section entitled "Non-printing
       characters" above for details. Perl also uses \N{name} to specify
       characters by Unicode name; PCRE2 does not support this.

       Each pair of lower and upper case escape sequences partitions the
       complete set of characters into two disjoint sets. Any given character
       matches one, and only one, of each pair. The sequences can appear both
       inside and outside character classes. They each match one character of
       the appropriate type. If the current matching point is at the end of
       the subject string, all of them fail, because there is no character to
       match.

       The default \s characters are HT (9), LF (10), VT (11), FF (12), CR
       (13), and space (32), which are defined as white space in the "C"
       locale. This list may vary if locale-specific matching is taking place.
       For example, in some locales the "non-breaking space" character (\xA0)
       is recognized as white space, and in others the VT character is not.

       A "word" character is an underscore or any character that is a letter
       or digit. By default, the definition of letters and digits is
       controlled by PCRE2's low-valued character tables, and may vary if
       locale-specific matching is taking place (see "Locale support" in the
       pcre2api page). For example, in a French locale such as "fr_FR" in
       Unix-like systems, or "french" in Windows, some character codes greater
       than 127 are used for accented letters, and these are then matched by
       \w. The use of locales with Unicode is discouraged.

       By default, characters whose code points are greater than 127 never
       match \d, \s, or \w, and always match \D, \S, and \W, although this may
       be different for characters in the range 128-255 when locale-specific
       matching is happening. These escape sequences retain their original
       meanings from before Unicode support was available, mainly for
       efficiency reasons. If the PCRE2_UCP option is set, the behaviour is
       changed so that Unicode properties are used to determine character
       types, as follows:

       \d  any character that matches \p{Nd} (decimal digit)
       \s  any character that matches \p{Z} or \h or \v
       \w  any character that matches \p{L}, \p{N}, \p{Mn}, or \p{Pc}

       The addition of \p{Mn} (non-spacing mark) and the replacement of an
       explicit test for underscore with a test for \p{Pc} (connector
       punctuation) happened in PCRE2 release 10.43. This brings PCRE2 into
       line with Perl.

       The upper case escapes match the inverse sets of characters. Note that
       \d matches only decimal digits, whereas \w matches any Unicode digit,
       as well as other character categories. Note also that PCRE2_UCP affects
       \b, and \B because they are defined in terms of \w and \W. Matching
       these sequences is noticeably slower when PCRE2_UCP is set.

       The effect of PCRE2_UCP on any one of these escape sequences can be
       negated by the options PCRE2_EXTRA_ASCII_BSD, PCRE2_EXTRA_ASCII_BSS,
       and PCRE2_EXTRA_ASCII_BSW, respectively. These options can be set and
       reset within a pattern by means of an internal option setting (see
       below).

       The sequences \h, \H, \v, and \V, in contrast to the other sequences,
       which match only ASCII characters by default, always match a specific
       list of code points, whether or not PCRE2_UCP is set. The horizontal
       space characters are:

       U+0009     Horizontal tab (HT)
       U+0020     Space
       U+00A0     Non-break space
       U+1680     Ogham space mark
       U+180E     Mongolian vowel separator
       U+2000     En quad
       U+2001     Em quad
       U+2002     En space
       U+2003     Em space
       U+2004     Three-per-em space
       U+2005     Four-per-em space
       U+2006     Six-per-em space
       U+2007     Figure space
       U+2008     Punctuation space
       U+2009     Thin space
       U+200A     Hair space
       U+202F     Narrow no-break space
       U+205F     Medium mathematical space
       U+3000     Ideographic space

       The vertical space characters are:

       U+000A     Linefeed (LF)
       U+000B     Vertical tab (VT)
       U+000C     Form feed (FF)
       U+000D     Carriage return (CR)
       U+0085     Next line (NEL)
       U+2028     Line separator
       U+2029     Paragraph separator

       In 8-bit, non-UTF-8 mode, only the characters with code points less
       than 256 are relevant.

   Newline sequences
       Outside a character class, by default, the escape sequence \R matches
       any Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent
       to the following:

       (?>\r\n|\n|\x0b|\f|\r|\x85)

       This is an example of an "atomic group", details of which are given
       below. This particular group matches either the two-character sequence
       CR followed by LF, or one of the single characters LF (linefeed,
       U+000A), VT (vertical tab, U+000B), FF (form feed, U+000C), CR
       (carriage return, U+000D), or NEL (next line, U+0085). Because this is
       an atomic group, the two-character sequence is treated as a single unit
       that cannot be split.

       In other modes, two additional characters whose code points are greater
       than 255 are added: LS (line separator, U+2028) and PS (paragraph
       separator, U+2029). Unicode support is not needed for these characters
       to be recognized.

       It is possible to restrict \R to match only CR, LF, or CRLF (instead of
       the complete set of Unicode line endings) by setting the option
       PCRE2_BSR_ANYCRLF at compile time. (BSR is an abbreviation for
       "backslash R".) This can be made the default when PCRE2 is built; if
       this is the case, the other behaviour can be requested via the
       PCRE2_BSR_UNICODE option. It is also possible to specify these settings
       by starting a pattern string with one of the following sequences:

       (*BSR_ANYCRLF)   CR, LF, or CRLF only
       (*BSR_UNICODE)   any Unicode newline sequence

       These override the default and the options given to the compiling
       function. Note that these special settings, which are not
       Perl-compatible, are recognized only at the very start of a pattern,
       and that they must be in upper case. If more than one of them is
       present, the last one is used. They can be combined with a change of
       newline convention; for example, a pattern can start with:

       (*ANY)(*BSR_ANYCRLF)

       They can also be combined with the (*UTF) or (*UCP) special sequences.
       Inside a character class, \R is treated as an unrecognized escape
       sequence, and causes an error.

   Unicode character properties
       When PCRE2 is built with Unicode support (the default), three
       additional escape sequences that match characters with specific
       properties are available. They can be used in any mode, though in 8-bit
       and 16-bit non-UTF modes these sequences are of course limited to
       testing characters whose code points are less than U+0100 or U+10000,
       respectively. In 32-bit non-UTF mode, code points greater than 0x10ffff
       (the Unicode limit) may be encountered. These are all treated as being
       in the Unknown script and with an unassigned type.

       Matching characters by Unicode property is not fast, because PCRE2 has
       to do a multistage table lookup in order to find a character's
       property. That is why the traditional escape sequences such as \d and
       \w do not use Unicode properties in PCRE2 by default, though you can
       make them do so by setting the PCRE2_UCP option or by starting the
       pattern with (*UCP).

       The extra escape sequences that provide property support are:

       \p{_xx_}   a character with the _xx_ property
       \P{_xx_}   a character without the _xx_ property
       \X       a Unicode extended grapheme cluster

       For compatibility with Perl, negation can be specified by including a
       circumflex between the opening brace and the property. For example,
       \p{^Lu} is the same as \P{Lu}.

       In accordance with Unicode's "loose matching" rules, ASCII white space
       characters, hyphens, and underscores are ignored in the properties
       represented by xx above. As well as the space character, ASCII white
       space can be tab, linefeed, vertical tab, formfeed, or carriage return.

       Some properties are specified as a name only; others as a name and a
       value, separated by a colon or an equals sign. The names and values
       consist of ASCII letters and digits (with one Perl-specific exception,
       see below). They are not case sensitive. Note, however, that the
       escapes themselves, \p and \P, are case sensitive. There are
       abbreviations for many names. The following examples are all
       equivalent:

       \p{bidiclass=al}
       \p{BC=al}
       \p{ Bidi_Class : AL }
       \p{ Bi-di class = Al }
       \P{ ^ Bi-di class = Al }

       There is support for Unicode script names, Unicode general category
       properties, "Any", which matches any character (including newline),
       Bidi_Class, a number of binary (yes/no) properties, and some special
       PCRE2 properties (described below). Certain other Perl properties such
       as "InMusicalSymbols" are not supported by PCRE2. Note that \P{Any}
       does not match any characters, so always causes a match failure.

   Script properties for \p and \P
       There are three different syntax forms for matching a script. Each
       Unicode character has a basic script and, optionally, a list of other
       scripts ("Script Extensions") with which it is commonly used. Using the
       Adlam script as an example, \p{sc:Adlam} matches characters whose basic
       script is Adlam, whereas \p{scx:Adlam} matches, in addition, characters
       that have Adlam in their extensions list. The full names "script" and
       "script extensions" for the property types are recognized and, as for
       all property specifications, an equals sign is an alternative to the
       colon. If a script name is given without a property type, for example,
       \p{Adlam}, it is treated as \p{scx:Adlam}. Perl changed to this
       interpretation at release 5.26 and PCRE2 changed at release 10.40.

       Unassigned characters (and in non-UTF 32-bit mode, characters with code
       points greater than 0x10FFFF) are assigned the "Unknown" script. Others
       that are not part of an identified script are lumped together as
       "Common". The current list of recognized script names and their
       4-character abbreviations can be obtained by running this command:

       pcre2test -LS

   The general category property for \p and \P
       Each character has exactly one Unicode general category property,
       specified by a two-letter abbreviation. If only one letter is specified
       with \p or \P, it includes all the general category properties that
       start with that letter. In this case, in the absence of negation, the
       curly brackets in the escape sequence are optional; these two examples
       have the same effect:

       \p{L}
       \pL

       The following general category property codes are supported:

       C     Other
       Cc    Control
       Cf    Format
       Cn    Unassigned
       Co    Private use
       Cs    Surrogate

       L     Letter
       Lc    Cased letter
       Ll    Lower case letter
       Lm    Modifier letter
       Lo    Other letter
       Lt    Title case letter
       Lu    Upper case letter

       M     Mark
       Mc    Spacing mark
       Me    Enclosing mark
       Mn    Non-spacing mark

       N     Number
       Nd    Decimal number
       Nl    Letter number
       No    Other number

       P     Punctuation
       Pc    Connector punctuation
       Pd    Dash punctuation
       Pe    Close punctuation
       Pf    Final punctuation
       Pi    Initial punctuation
       Po    Other punctuation
       Ps    Open punctuation

       S     Symbol
       Sc    Currency symbol
       Sk    Modifier symbol
       Sm    Mathematical symbol
       So    Other symbol

       Z     Separator
       Zl    Line separator
       Zp    Paragraph separator
       Zs    Space separator

       Perl originally used the name L& for the Lc property. This is still
       supported by Perl, but discouraged. PCRE2 also still supports it. This
       property matches any character that has the Lu, Ll, or Lt property, in
       other words, any letter that is not classified as a modifier or
       "other". From release 10.45 of PCRE2 the properties Lu, Ll, and Lt are
       all treated as Lc when case-independent matching is set by the
       PCRE2_CASELESS option or (?i) within the pattern. The other properties
       are not affected by caseless matching.

       The Cs (Surrogate) property applies only to characters whose code
       points are in the range U+D800 to U+DFFF. These characters are no
       different to any other character when PCRE2 is not in UTF mode (using
       the 16-bit or 32-bit library). However, they are not valid in Unicode
       strings and so cannot be tested by PCRE2 in UTF mode, unless UTF
       validity checking has been turned off (see the discussion of
       PCRE2_NO_UTF_CHECK in the pcre2api page).

       The long synonyms for property names that Perl supports (such as
       \p{Letter}) are not supported by PCRE2, nor is it permitted to prefix
       any of these properties with "Is".

       No character that is in the Unicode table has the Cn (unassigned)
       property. Instead, this property is assumed for any code point that is
       not in the Unicode table.

   Binary (yes/no) properties for \p and \P
       Unicode defines a number of binary properties, that is, properties
       whose only values are true or false. You can obtain a list of those
       that are recognized by \p and \P, along with their abbreviations, by
       running this command:

       pcre2test -LP

   The Bidi_Class property for \p and \P
       \p{Bidi_Class:<class>}   matches a character with the given class
       \p{BC:<class>}           matches a character with the given class

       The recognized classes are:

       AL          Arabic letter
       AN          Arabic number
       B           paragraph separator
       BN          boundary neutral
       CS          common separator
       EN          European number
       ES          European separator
       ET          European terminator
       FSI         first strong isolate
       L           left-to-right
       LRE         left-to-right embedding
       LRI         left-to-right isolate
       LRO         left-to-right override
       NSM         non-spacing mark
       ON          other neutral
       PDF         pop directional format
       PDI         pop directional isolate
       R           right-to-left
       RLE         right-to-left embedding
       RLI         right-to-left isolate
       RLO         right-to-left override
       S           segment separator
       WS          white space

       As in all property specifications, an equals sign may be used instead
       of a colon and the class names are case-insensitive. Only the short
       names listed above are recognized; PCRE2 does not at present support
       any long alternatives.

   Extended grapheme clusters
       The \X escape matches any number of Unicode characters that form an
       "extended grapheme cluster", and treats the sequence as an atomic group
       (see below). Unicode supports various kinds of composite character by
       giving each character a grapheme breaking property, and having rules
       that use these properties to define the boundaries of extended grapheme
       clusters. The rules are defined in Unicode Standard Annex 29, "Unicode
       Text Segmentation". Unicode 11.0.0 abandoned the use of some previous
       properties that had been used for emojis. Instead it introduced various
       emoji-specific properties. PCRE2 uses only the Extended Pictographic
       property.

       \X always matches at least one character. Then it decides whether to
       add additional characters according to the following rules for ending a
       cluster:

        1. End at the end of the subject string.

        2. Do not end between CR and LF; otherwise end after any control
           character.

        3. Do not break Hangul (a Korean script) syllable sequences. Hangul
           characters are of five types: L, V, T, LV, and LVT. An L character
           may be followed by an L, V, LV, or LVT character; an LV or V
           character may be followed by a V or T character; an LVT or T
           character may be followed only by a T character.

        4. Do not end before extending characters or spacing marks or the
           zero-width joiner (ZWJ) character. Characters with the "mark"
           property always have the "extend" grapheme breaking property.

        5. Do not end after prepend characters.

        6. Do not end within emoji modifier sequences or emoji ZWJ (zero-width
           joiner) sequences. An emoji ZWJ sequence consists of a character
           with the Extended_Pictographic property, optionally followed by one
           or more characters with the Extend property, followed by the ZWJ
           character, followed by another Extended_Pictographic character.

        7. Do not break within emoji flag sequences. That is, do not break
           between regional indicator (RI) characters if there are an odd
           number of RI characters before the break point.

        8. Otherwise, end the cluster.

   PCRE2's additional properties
       As well as the standard Unicode properties described above, PCRE2
       supports four more that make it possible to convert traditional escape
       sequences such as \w and \s to use Unicode properties. PCRE2 uses these
       non-standard, non-Perl properties internally when PCRE2_UCP is set.
       However, they may also be used explicitly. These properties are:

       Xan   Any alphanumeric character
       Xps   Any POSIX space character
       Xsp   Any Perl space character
       Xwd   Any Perl "word" character

       Xan matches characters that have either the L (letter) or the N
       (number) property. Xps matches the characters tab, linefeed, vertical
       tab, form feed, or carriage return, and any other character that has
       the Z (separator) property (this includes the space character). Xsp is
       the same as Xps; in PCRE1 it used to exclude vertical tab, for Perl
       compatibility, but Perl changed. Xwd matches the same characters as
       Xan, plus those that match Mn (non-spacing mark) or Pc (connector
       punctuation, which includes underscore).

       There is another non-standard property, Xuc, which matches any
       character that can be represented by a Universal Character Name in C++
       and other programming languages. These are the characters $, @, `
       (grave accent), and all characters with Unicode code points greater
       than or equal to U+00A0, except for the surrogates U+D800 to U+DFFF.
       Note that most base (ASCII) characters are excluded. (Universal
       Character Names are of the form \uHHHH or \UHHHHHHHH where H is a
       hexadecimal digit. Note that the Xuc property does not match these
       sequences but the characters that they represent.)

   Resetting the match start
       In normal use, the escape sequence \K causes any previously matched
       characters not to be included in the final matched sequence that is
       returned. For example, the pattern:

       foo\Kbar

       matches "foobar", but reports that it has matched "bar". \K does not
       interact with anchoring in any way. The pattern:

       ^foo\Kbar

       matches only when the subject begins with "foobar" (in single line
       mode), though it again reports the matched string as "bar". This
       feature is similar to a lookbehind assertion (described below), but the
       part of the pattern that precedes \K is not constrained to match a
       limited number of characters, as is required for a lookbehind
       assertion. The use of \K does not interfere with the setting of
       captured substrings. For example, when the pattern

       (foo)\Kbar

       matches "foobar", the first substring is still set to "foo".

       From version 5.32.0 Perl forbids the use of \K in lookaround
       assertions. From release 10.38 PCRE2 also forbids this by default.
       However, the PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK option can be used when
       calling pcre2_compile() to re-enable the previous behaviour. When this
       option is set, \K is acted upon when it occurs inside positive
       assertions, but is ignored in negative assertions. Note that when a
       pattern such as (?=ab\K) matches, the reported start of the match can
       be greater than the end of the match. Using \K in a lookbehind
       assertion at the start of a pattern can also lead to odd effects. For
       example, consider this pattern:

       (?<=\Kfoo)bar

       If the subject is "foobar", a call to pcre2_match() with a starting
       offset of 3 succeeds and reports the matching string as "foobar", that
       is, the start of the reported match is earlier than where the match
       started.

   Simple assertions
       The final use of backslash is for certain simple assertions. An
       assertion specifies a condition that has to be met at a particular
       point in a match, without consuming any characters from the subject
       string. The use of groups for more complicated assertions is described
       below. The backslashed assertions are:

       \b     matches at a word boundary
       \B     matches when not at a word boundary
       \A     matches at the start of the subject
       \Z     matches at the end of the subject
               also matches before a newline at the end of the subject
       \z     matches only at the end of the subject
       \G     matches at the first matching position in the subject

       Inside a character class, \b has a different meaning; it matches the
       backspace character. If any other of these assertions appears in a
       character class, an "invalid escape sequence" error is generated.

       A word boundary is a position in the subject string where the current
       character and the previous character do not both match \w or \W (i.e.
       one matches \w and the other matches \W), or the start or end of the
       string if the first or last character matches \w, respectively. When
       PCRE2 is built with Unicode support, the meanings of \w and \W can be
       changed by setting the PCRE2_UCP option. When this is done, it also
       affects \b and \B. Neither PCRE2 nor Perl has a separate "start of
       word" or "end of word" metasequence. However, whatever follows \b
       normally determines which it is. For example, the fragment \ba matches
       "a" at the start of a word.

       The \A, \Z, and \z assertions differ from the traditional circumflex
       and dollar (described in the next section) in that they only ever match
       at the very start and end of the subject string, whatever options are
       set. Thus, they are independent of multiline mode. These three
       assertions are not affected by the PCRE2_NOTBOL or PCRE2_NOTEOL
       options, which affect only the behaviour of the circumflex and dollar
       metacharacters. However, if the startoffset argument of pcre2_match()
       is non-zero, indicating that matching is to start at a point other than
       the beginning of the subject, \A can never match. The difference
       between \Z and \z is that \Z matches before a newline at the end of the
       string as well as at the very end, whereas \z matches only at the end.

       The \G assertion is true only when the current matching position is at
       the start point of the matching process, as specified by the
       startoffset argument of pcre2_match(). It differs from \A when the
       value of startoffset is non-zero. By calling pcre2_match() multiple
       times with appropriate arguments, you can mimic Perl's /g option, and
       it is in this kind of implementation where \G can be useful.

       Note, however, that PCRE2's implementation of \G, being true at the
       starting character of the matching process, is subtly different from
       Perl's, which defines it as true at the end of the previous match. In
       Perl, these can be different when the previously matched string was
       empty. Because PCRE2 does just one match at a time, it cannot reproduce
       this behaviour.

       If all the alternatives of a pattern begin with \G, the expression is
       anchored to the starting match position, and the "anchored" flag is set
       in the compiled regular expression.


CIRCUMFLEX AND DOLLAR
       The circumflex and dollar metacharacters are zero-width assertions.
       That is, they test for a particular condition being true without
       consuming any characters from the subject string. These two
       metacharacters are concerned with matching the starts and ends of
       lines. If the newline convention is set so that only the two-character
       sequence CRLF is recognized as a newline, isolated CR and LF characters
       are treated as ordinary data characters, and are not recognized as
       newlines.

       Outside a character class, in the default matching mode, the circumflex
       character is an assertion that is true only if the current matching
       point is at the start of the subject string. If the startoffset
       argument of pcre2_match() is non-zero, or if PCRE2_NOTBOL is set,
       circumflex can never match if the PCRE2_MULTILINE option is unset.
       Inside a character class, circumflex has an entirely different meaning
       (see below).

       Circumflex need not be the first character of the pattern if a number
       of alternatives are involved, but it should be the first thing in each
       alternative in which it appears if the pattern is ever to match that
       branch. If all possible alternatives start with a circumflex, that is,
       if the pattern is constrained to match only at the start of the
       subject, it is said to be an "anchored" pattern. (There are also other
       constructs that can cause a pattern to be anchored.)

       The dollar character is an assertion that is true only if the current
       matching point is at the end of the subject string, or immediately
       before a newline at the end of the string (by default), unless
       PCRE2_NOTEOL is set. Note, however, that it does not actually match the
       newline. Dollar need not be the last character of the pattern if a
       number of alternatives are involved, but it should be the last item in
       any branch in which it appears. Dollar has no special meaning in a
       character class.

       The meaning of dollar can be changed so that it matches only at the
       very end of the string, by setting the PCRE2_DOLLAR_ENDONLY option at
       compile time. This does not affect the \Z assertion.

       The meanings of the circumflex and dollar metacharacters are changed if
       the PCRE2_MULTILINE option is set. When this is the case, a dollar
       character matches before any newlines in the string, as well as at the
       very end, and a circumflex matches immediately after internal newlines
       as well as at the start of the subject string. It does not match after
       a newline that ends the string, for compatibility with Perl. However,
       this can be changed by setting the PCRE2_ALT_CIRCUMFLEX option.

       For example, the pattern /^abc$/ matches the subject string "def\nabc"
       (where \n represents a newline) in multiline mode, but not otherwise.
       Consequently, patterns that are anchored in single line mode because
       all branches start with ^ are not anchored in multiline mode, and a
       match for circumflex is possible when the startoffset argument of
       pcre2_match() is non-zero. The PCRE2_DOLLAR_ENDONLY option is ignored
       if PCRE2_MULTILINE is set.

       When the newline convention (see "Newline conventions" below)
       recognizes the two-character sequence CRLF as a newline, this is
       preferred, even if the single characters CR and LF are also recognized
       as newlines. For example, if the newline convention is "any", a
       multiline mode circumflex matches before "xyz" in the string
       "abc\r\nxyz" rather than after CR, even though CR on its own is a valid
       newline. (It also matches at the very start of the string, of course.)

       Note that the sequences \A, \Z, and \z can be used to match the start
       and end of the subject in both modes, and if all branches of a pattern
       start with \A it is always anchored, whether or not PCRE2_MULTILINE is
       set.


FULL STOP (PERIOD, DOT) AND \N
       Outside a character class, a dot in the pattern matches any one
       character in the subject string except (by default) a character that
       signifies the end of a line. One or more characters may be specified as
       line terminators (see "Newline conventions" above).

       Dot never matches a single line-ending character. When the
       two-character sequence CRLF is the only line ending, dot does not match
       CR if it is immediately followed by LF, but otherwise it matches all
       characters (including isolated CRs and LFs). When ANYCRLF is selected
       for line endings, no occurrences of CR of LF match dot. When all
       Unicode line endings are being recognized, dot does not match CR or LF
       or any of the other line ending characters.

       The behaviour of dot with regard to newlines can be changed. If the
       PCRE2_DOTALL option is set, a dot matches any one character, without
       exception. If the two-character sequence CRLF is present in the subject
       string, it takes two dots to match it.

       The handling of dot is entirely independent of the handling of
       circumflex and dollar, the only relationship being that they both
       involve newlines. Dot has no special meaning in a character class.

       The escape sequence \N when not followed by an opening brace behaves
       like a dot, except that it is not affected by the PCRE2_DOTALL option.
       In other words, it matches any character except one that signifies the
       end of a line.

       When \N is followed by an opening brace it has a different meaning. See
       the section entitled "Non-printing characters" above for details. Perl
       also uses \N{name} to specify characters by Unicode name; PCRE2 does
       not support this.


MATCHING A SINGLE CODE UNIT
       Outside a character class, the escape sequence \C matches any one code
       unit, whether or not a UTF mode is set. In the 8-bit library, one code
       unit is one byte; in the 16-bit library it is a 16-bit unit; in the
       32-bit library it is a 32-bit unit. Unlike a dot, \C always matches
       line-ending characters. The feature is provided in Perl in order to
       match individual bytes in UTF-8 mode, but it is unclear how it can
       usefully be used.

       Because \C breaks up characters into individual code units, matching
       one unit with \C in UTF-8 or UTF-16 mode means that the rest of the
       string may start with a malformed UTF character. This has undefined
       results, because PCRE2 assumes that it is matching character by
       character in a valid UTF string (by default it checks the subject
       string's validity at the start of processing unless the
       PCRE2_NO_UTF_CHECK or PCRE2_MATCH_INVALID_UTF option is used).

       An application can lock out the use of \C by setting the
       PCRE2_NEVER_BACKSLASH_C option when compiling a pattern. It is also
       possible to build PCRE2 with the use of \C permanently disabled.

       PCRE2 does not allow \C to appear in lookbehind assertions (described
       below) in UTF-8 or UTF-16 modes, because this would make it impossible
       to calculate the length of the lookbehind. Neither the alternative
       matching function pcre2_dfa_match() nor the JIT optimizer support \C in
       these UTF modes. The former gives a match-time error; the latter fails
       to optimize and so the match is always run using the interpreter.

       In the 32-bit library, however, \C is always supported (when not
       explicitly locked out) because it always matches a single code unit,
       whether or not UTF-32 is specified.

       In general, the \C escape sequence is best avoided. However, one way of
       using it that avoids the problem of malformed UTF-8 or UTF-16
       characters is to use a lookahead to check the length of the next
       character, as in this pattern, which could be used with a UTF-8 string
       (ignore white space and line breaks):

       (?| (?=[\x00-\x7f])(\C) |
           (?=[\x80-\x{7ff}])(\C)(\C) |
           (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
           (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))

       In this example, a group that starts with (?| resets the capturing
       parentheses numbers in each alternative (see "Duplicate Group Numbers"
       below). The assertions at the start of each branch check the next UTF-8
       character for values whose encoding uses 1, 2, 3, or 4 bytes,
       respectively. The character's individual bytes are then captured by the
       appropriate number of \C groups.


SQUARE BRACKETS AND CHARACTER CLASSES
       An opening square bracket introduces a character class, terminated by a
       closing square bracket. A closing square bracket on its own is not
       special by default. If a closing square bracket is required as a member
       of the class, it should be the first data character in the class (after
       an initial circumflex, if present) or escaped with a backslash. This
       means that, by default, an empty class cannot be defined. However, if
       the PCRE2_ALLOW_EMPTY_CLASS option is set, a closing square bracket at
       the start does end the (empty) class.

       A character class matches a single character in the subject. A matched
       character must be in the set of characters defined by the class, unless
       the first character in the class definition is a circumflex, in which
       case the subject character must not be in the set defined by the class.
       If a circumflex is actually required as a member of the class, ensure
       it is not the first character, or escape it with a backslash.

       For example, the character class [aeiou] matches any lower case English
       vowel, whereas [^aeiou] matches all other characters. Note that a
       circumflex is just a convenient notation for specifying the characters
       that are in the class by enumerating those that are not. A class that
       starts with a circumflex is not an assertion; it still consumes a
       character from the subject string, and therefore it fails to match if
       the current pointer is at the end of the string.

       Characters in a class may be specified by their code points using \o,
       \x, or \N{U+hh..} in the usual way. When caseless matching is set, any
       letters in a class represent both their upper case and lower case
       versions, so for example, a caseless [aeiou] matches "A" as well as
       "a", and a caseless [^aeiou] does not match "A", whereas a caseful
       version would. Note that there are two ASCII characters, K and S, that,
       in addition to their lower case ASCII equivalents, are case-equivalent
       with Unicode U+212A (Kelvin sign) and U+017F (long S) respectively when
       either PCRE2_UTF or PCRE2_UCP is set. If you do not want these
       ASCII/non-ASCII case equivalences, you can suppress them by setting
       PCRE2_EXTRA_CASELESS_RESTRICT, either as an option in a compile
       context, or by including (*CASELESS_RESTRICT) or (?r) within a pattern.

       Characters that might indicate line breaks are never treated in any
       special way when matching character classes, whatever line-ending
       sequence is in use, and whatever setting of the PCRE2_DOTALL and
       PCRE2_MULTILINE options is used. A class such as [^a] always matches
       one of these characters.

       The generic character type escape sequences \d, \D, \h, \H, \p, \P, \s,
       \S, \v, \V, \w, and \W may appear in a character class, and add the
       characters that they match to the class. For example, [\dABCDEF]
       matches any hexadecimal digit. In UTF modes, the PCRE2_UCP option
       affects the meanings of \d, \s, \w and their upper case partners, just
       as it does when they appear outside a character class, as described in
       the section entitled "Generic character types" above. The escape
       sequence \b has a different meaning inside a character class; it
       matches the backspace character. The sequences \B, \R, and \X are not
       special inside a character class. Like any other unrecognized escape
       sequences, they cause an error. The same is true for \N when not
       followed by an opening brace.

       The minus (hyphen) character can be used to specify a range of
       characters in a character class. For example, [d-m] matches any letter
       between d and m, inclusive. If a minus character is required in a
       class, it must be escaped with a backslash or appear in a position
       where it cannot be interpreted as indicating a range, typically as the
       first or last character in the class, or immediately after a range. For
       example, [b-d-z] matches letters in the range b to d, a hyphen
       character, or z.

       There is some special treatment for alphabetic ranges in EBCDIC
       environments; see the section "EBCDIC environments" below.

       Perl treats a hyphen as a literal if it appears before or after a POSIX
       class (see below) or before or after a character type escape such as \d
       or \H. However, unless the hyphen is the last character in the class,
       Perl outputs a warning in its warning mode, as this is most likely a
       user error. As PCRE2 has no facility for warning, an error is given in
       these cases.

       It is not possible to have the literal character "]" as the end
       character of a range. A pattern such as [W-]46] is interpreted as a
       class of two characters ("W" and "-") followed by a literal string
       "46]", so it would match "W46]" or "-46]". However, if the "]" is
       escaped with a backslash it is interpreted as the end of a range, so
       [W-\]46] is interpreted as a class containing a range and two other
       characters. The octal or hexadecimal representation of "]" can also be
       used to end a range.

       Ranges normally include all code points between the start and end
       characters, inclusive. They can also be used for code points specified
       numerically, for example [\000-\037]. Ranges can include any characters
       that are valid for the current mode. In any UTF mode, the so-called
       "surrogate" characters (those whose code points lie between 0xd800 and
       0xdfff inclusive) may not be specified explicitly by default (the
       PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES option disables this check).
       However, ranges such as [\x{d7ff}-\x{e000}], which include the
       surrogates, are always permitted.

       If a range that includes letters is used when caseless matching is set,
       it matches the letters in either case. For example, [W-c] is equivalent
       to [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if
       character tables for a French locale are in use, [\xc8-\xcb] matches
       accented E characters in both cases.

       A circumflex can conveniently be used with the upper case character
       types to specify a more restricted set of characters than the matching
       lower case type. For example, the class [^\W_] matches any letter or
       digit, but not underscore, whereas [\w] includes underscore. A positive
       character class should be read as "something OR something OR ..." and a
       negative class as "NOT something AND NOT something AND NOT ...".

       The metacharacters that are recognized in character classes are
       backslash, hyphen (when it can be interpreted as specifying a range),
       circumflex (only at the start), and the terminating closing square
       bracket. An opening square bracket is also special when it can be
       interpreted as introducing a POSIX class (see "Posix character classes"
       below), or a special compatibility feature (see "Compatibility feature
       for word boundaries" below. Escaping any non-alphanumeric character in
       a class turns it into a literal, whether or not it would otherwise be a
       metacharacter.


PERL EXTENDED CHARACTER CLASSES
       From release 10.45 PCRE2 supports Perl's (?[...]) extended character
       class syntax. This can be used to perform set operations such as
       intersection on character classes.

       The syntax permitted within (?[...]) is quite different to ordinary
       character classes. Inside the extended class, there is an expression
       syntax consisting of "atoms", operators, and ordinary parentheses "()"
       used for grouping. Such classes always have the Perl /xx modifier
       (PCRE2 option PCRE2_EXTENDED_MORE) turned on within them. This means
       that literal space and tab characters are ignored everywhere in the
       class.

       The allowed atoms are individual characters specified by escape
       sequences such as \n or \x{123}, character types such as \d, POSIX
       classes such as [:alpha:], and nested ordinary (non-extended) character
       classes. For example, in (?[\d & [...]]) the nested class [...] follows
       the usual rules for ordinary character classes, in which parentheses
       are not metacharacters, and character literals and ranges are
       permitted.

       Character literals and ranges may not appear outside a nested ordinary
       character class because they are not atoms in the extended syntax. The
       extended syntax does not introduce any additional escape sequences, so
       (?[\y]) is an unknown escape, as it would be in [\y].

       In the extended syntax, ^ does not negate a class (except within an
       ordinary class nested inside an extended class); it is instead a binary
       operator.

       The binary operators are "&" (intersection), "|" or + (union), "-"
       (subtraction) and "^" (symmetric difference). These are
       left-associative and "&" has higher (tighter) precedence, while the
       others have equal lower precedence. The one prefix unary operator is
       "!" (complement), with highest precedence.


UTS#18 EXTENDED CHARACTER CLASSES
       The PCRE2_ALT_EXTENDED_CLASS option enables an alternative to Perl's
       (?[...]) syntax, allowing instead extended class behaviour inside
       ordinary [...] character classes. This altered syntax for [...] classes
       is loosely described by the Unicode standard UTS#18. The
       PCRE2_ALT_EXTENDED_CLASS option does not prevent use of (?[...])
       classes; it just changes the meaning of all [...] classes that are not
       nested inside a Perl (?[...]) class.

       Firstly, in ordinary Perl [...] syntax, an expression such as "[a[]" is
       a character class with two literal characters "a" and "[", but in
       UTS#18 extended classes the "[" character becomes an additional
       metacharacter within classes, denoting the start of a nested class, so
       a literal "[" must be escaped as "\[".

       Secondly, within the UTS#18 extended syntax, there are operators "||",
       "&&", "--" and "~~" which denote character class union, intersection,
       subtraction, and symmetric difference respectively. In standard Perl
       syntax, these would simply be needlessly-repeated literals (except for
       "--" which could be the start or end of a range). In UTS#18 extended
       classes these operators can be used in constructs such as [\p{L}--[QW]]
       for "Unicode letters, other than Q and W". A literal "-" at the start
       or end of a range must be escaped, so while "[--1]" in Perl syntax is
       the range from hyphen to "1", it must be escaped as "[\--1]" in UTS#18
       extended classes.

       Unlike Perl's (?[...]) extended classes, the PCRE2_EXTENDED_MORE option
       to ignore space and tab characters is not automatically enabled for
       UTS#18 extended classes, but it is honoured if set.

       Extended UTS#18 classes can be nested, and nested classes are
       themselves extended classes (unlike Perl, where nested classes must be
       simple classes). For example, [\p{L}&&[\p{Thai}||\p{Greek}]] matches
       any letter that is in the Thai or Greek scripts. Note that this means
       that no special grouping characters (such as the parentheses used in
       Perl's (?[...]) class syntax) are needed.

       Individual class items (literal characters, literal ranges, properties
       such as \d or \p{...}, and nested classes) can be combined by
       juxtaposition or by an operator. Juxtaposition is the implicit union
       operator, and binds more tightly than any explicit operator. Thus a
       sequence of literals and/or ranges behaves as if it is enclosed in
       square brackets. For example, [A-Z0-9&&[^E8]] is the same as
       [[A-Z0-9]&&[^E8]], which matches any upper case alphanumeric character
       except "E" or "8".

       Precedence between the explicit operators is not defined, so mixing
       operators is a syntax error. For example, [A&&B--C] is an error, but
       [A&&[B--C]] is valid.

       This is an emerging syntax which is being adopted gradually across the
       regex ecosystem: for example JavaScript adopted the "/v" flag in
       ECMAScript 2024; Python's "re" module reserves the syntax for future
       use with a FutureWarning for unescaped use of "[" as a literal within
       character classes. Due to UTS#18 providing insufficient guidance,
       engines interpret the syntax differently. Rust's "regex" crate and
       Python's "regex" PyPi module both implement UTS#18 extended classes,
       but with slight incompatibilities ([A||B&&C] is parsed as [A||[B&&C]]
       in Python's "regex" but as [[A||B]&&C] in Rust's "regex").

       PCRE2's syntax adds syntax restrictions similar to ECMASCript's /v
       flag, so that all the UTS#18 extended classes accepted as valid by
       PCRE2 have the property that they are interpreted either with the same
       behaviour, or as invalid, by all other major engines. Please file an
       issue if you are aware of cross-engine differences in behaviour between
       PCRE2 and another major engine.


POSIX CHARACTER CLASSES
       Perl supports the POSIX notation for character classes. This uses names
       enclosed by [: and :] within the enclosing square brackets. PCRE2 also
       supports this notation, in both ordinary and extended classes. For
       example,

       [01[:alpha:]%]

       matches "0", "1", any alphabetic character, or "%". The supported class
       names are:

       alnum    letters and digits
       alpha    letters
       ascii    character codes 0 - 127
       blank    space or tab only
       cntrl    control characters
       digit    decimal digits (same as \d)
       graph    printing characters, excluding space
       lower    lower case letters
       print    printing characters, including space
       punct    printing characters, excluding letters and digits and space
       space    white space (the same as \s from PCRE2 8.34)
       upper    upper case letters
       word     "word" characters (same as \w)
       xdigit   hexadecimal digits

       The default "space" characters are HT (9), LF (10), VT (11), FF (12),
       CR (13), and space (32). If locale-specific matching is taking place,
       the list of space characters may be different; there may be fewer or
       more of them. "Space" and \s match the same set of characters, as do
       "word" and \w.

       The name "word" is a Perl extension, and "blank" is a GNU extension
       from Perl 5.8. Another Perl extension is negation, which is indicated
       by a ^ character after the colon. For example,

       [12[:^digit:]]

       matches "1", "2", or any non-digit. PCRE2 (and Perl) also recognize the
       POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
       these are not supported, and an error is given if they are encountered.

       By default, characters with values greater than 127 do not match any of
       the POSIX character classes, although this may be different for
       characters in the range 128-255 when locale-specific matching is
       happening. However, in UCP mode, unless certain options are set (see
       below), some of the classes are changed so that Unicode character
       properties are used. This is achieved by replacing POSIX classes with
       other sequences, as follows:

       [:alnum:]  becomes  \p{Xan}
       [:alpha:]  becomes  \p{L}
       [:blank:]  becomes  \h
       [:cntrl:]  becomes  \p{Cc}
       [:digit:]  becomes  \p{Nd}
       [:lower:]  becomes  \p{Ll}
       [:space:]  becomes  \p{Xps}
       [:upper:]  becomes  \p{Lu}
       [:word:]   becomes  \p{Xwd}

       Negated versions, such as [:^alpha:] use \P instead of \p. Four other
       POSIX classes are handled specially in UCP mode:

       [:graph:]  This matches characters that have glyphs that mark the page
                  when printed. In Unicode property terms, it matches all
                  characters with the L, M, N, P, S, or Cf properties, except
                  for:

                  U+061C           Arabic Letter Mark
                  U+180E           Mongolian Vowel Separator
                  U+2066 - U+2069  Various "isolate"s

       [:print:]  This matches the same characters as [:graph:] plus space
                  characters that are not controls, that is, characters with
                  the Zs property.

       [:punct:]  This matches all characters that have the Unicode P
                  (punctuation) property, plus those characters with code
                  points less than 256 that have the S (Symbol) property.

       [:xdigit:] In addition to the ASCII hexadecimal digits, this also
                  matches the "fullwidth" versions of those characters, whose
                  Unicode code points start at U+FF10. This is a change that
                  was made in PCRE2 release 10.43 for Perl compatibility.

       The other POSIX classes are unchanged by PCRE2_UCP, and match only
       characters with code points less than 256.

       There are two options that can be used to restrict the POSIX classes to
       ASCII characters when PCRE2_UCP is set. The option
       PCRE2_EXTRA_ASCII_DIGIT affects just [:digit:] and [:xdigit:]. Within a
       pattern, this can be set and unset by (?aT) and (?-aT). The
       PCRE2_EXTRA_ASCII_POSIX option disables UCP processing for all POSIX
       classes, including [:digit:] and [:xdigit:]. Within a pattern, (?aP)
       and (?-aP) set and unset both these options for consistency.


COMPATIBILITY FEATURE FOR WORD BOUNDARIES
       In the POSIX.2 compliant library that was included in 4.4BSD Unix, the
       ugly syntax [[:<:]] and [[:>:]] is used for matching "start of word"
       and "end of word". PCRE2 treats these items as follows:

       [[:<:]]  is converted to  \b(?=\w)
       [[:>:]]  is converted to  \b(?<=\w)

       Only these exact character sequences are recognized. A sequence such as
       [a[:<:]b] provokes error for an unrecognized POSIX class name. This
       support is not compatible with Perl. It is provided to help migrations
       from other environments, and is best not used in any new patterns. Note
       that \b matches at the start and the end of a word (see "Simple
       assertions" above), and in a Perl-style pattern the preceding or
       following character normally shows which is wanted, without the need
       for the assertions that are used above in order to give exactly the
       POSIX behaviour. Note also that the PCRE2_UCP option changes the
       meaning of \w (and therefore \b) by default, so it also affects these
       POSIX sequences.


VERTICAL BAR
       Vertical bar characters are used to separate alternative patterns. For
       example, the pattern

       gilbert|sullivan

       matches either "gilbert" or "sullivan". Any number of alternatives may
       appear, and an empty alternative is permitted (matching the empty
       string). The matching process tries each alternative in turn, from left
       to right, and the first one that succeeds is used. If the alternatives
       are within a group (defined below), "succeeds" means matching the rest
       of the main pattern as well as the alternative in the group.


INTERNAL OPTION SETTING
       The settings of several options can be changed within a pattern by a
       sequence of letters enclosed between "(?" and ")". The following are
       Perl-compatible, and are described in detail in the pcre2api
       documentation. The option letters are:

       i  for PCRE2_CASELESS
       m  for PCRE2_MULTILINE
       n  for PCRE2_NO_AUTO_CAPTURE
       s  for PCRE2_DOTALL
       x  for PCRE2_EXTENDED
       xx for PCRE2_EXTENDED_MORE

       For example, (?im) sets caseless, multiline matching. It is also
       possible to unset these options by preceding the relevant letters with
       a hyphen, for example (?-im). The two "extended" options are not
       independent; unsetting either one cancels the effects of both of them.

       A combined setting and unsetting such as (?im-sx), which sets
       PCRE2_CASELESS and PCRE2_MULTILINE while unsetting PCRE2_DOTALL and
       PCRE2_EXTENDED, is also permitted. Only one hyphen may appear in the
       options string. If a letter appears both before and after the hyphen,
       the option is unset. An empty options setting "(?)" is allowed.
       Needless to say, it has no effect.

       If the first character following (? is a circumflex, it causes all of
       the above options to be unset. Letters may follow the circumflex to
       cause some options to be re-instated, but a hyphen may not appear.

       Some PCRE2-specific options can be changed by the same mechanism using
       these pairs or individual letters:

       aD for PCRE2_EXTRA_ASCII_BSD
       aS for PCRE2_EXTRA_ASCII_BSS
       aW for PCRE2_EXTRA_ASCII_BSW
       aP for PCRE2_EXTRA_ASCII_POSIX and PCRE2_EXTRA_ASCII_DIGIT
       aT for PCRE2_EXTRA_ASCII_DIGIT
       r  for PCRE2_EXTRA_CASELESS_RESTRICT
       J  for PCRE2_DUPNAMES
       U  for PCRE2_UNGREEDY

       However, except for 'r', these are not unset by (?^), which is
       equivalent to (?-imnrsx). If 'a' is not followed by any of the upper
       case letters shown above, it sets (or unsets) all the ASCII options.

       PCRE2_EXTRA_ASCII_DIGIT has no additional effect when
       PCRE2_EXTRA_ASCII_POSIX is set, but including it in (?aP) means that
       (?-aP) suppresses all ASCII restrictions for POSIX classes.

       When one of these option changes occurs at top level (that is, not
       inside group parentheses), the change applies until a subsequent
       change, or the end of the pattern. An option change within a group (see
       below for a description of groups) affects only that part of the group
       that follows it. At the end of the group these options are reset to the
       state they were before the group. For example,

       (a(?i)b)c

       matches abc and aBc and no other strings (assuming PCRE2_CASELESS is
       not set externally). Any changes made in one alternative do carry on
       into subsequent branches within the same group. For example,

       (a(?i)b|c)

       matches "ab", "aB", "c", and "C", even though when matching "C" the
       first branch is abandoned before the option setting. This is because
       the effects of option settings happen at compile time. There would be
       some very weird behaviour otherwise.

       As a convenient shorthand, if any option settings are required at the
       start of a non-capturing group (see the next section), the option
       letters may appear between the "?" and the ":". Thus the two patterns

       (?i:saturday|sunday)
       (?:(?i)saturday|sunday)

       match exactly the same set of strings.

       Note: There are other PCRE2-specific options, applying to the whole
       pattern, which can be set by the application when the compiling
       function is called. In addition, the pattern can contain special
       leading sequences such as (*CRLF) to override what the application has
       set or what has been defaulted. Details are given in the section
       entitled "Newline sequences" above. There are also the (*UTF) and
       (*UCP) leading sequences that can be used to set UTF and Unicode
       property modes; they are equivalent to setting the PCRE2_UTF and
       PCRE2_UCP options, respectively. However, the application can set the
       PCRE2_NEVER_UTF or PCRE2_NEVER_UCP options, which lock out the use of
       the (*UTF) and (*UCP) sequences.


GROUPS
       Groups are delimited by parentheses (round brackets), which can be
       nested. Turning part of a pattern into a group does two things:

        1. It localizes a set of alternatives. For example, the pattern

       cat(aract|erpillar|)

       matches "cataract", "caterpillar", or "cat". Without the parentheses,
       it would match "cataract", "erpillar" or an empty string.

        1. It creates a "capture group". This means that, when the whole
           pattern matches, the portion of the subject string that matched the
           group is passed back to the caller, separately from the portion
           that matched the whole pattern. (This applies only to the
           traditional matching function; the DFA matching function does not
           support capturing.)

       Opening parentheses are counted from left to right (starting from 1) to
       obtain numbers for capture groups. For example, if the string "the red
       king" is matched against the pattern

       the ((red|white) (king|queen))

       the captured substrings are "red king", "red", and "king", and are
       numbered 1, 2, and 3, respectively.

       The fact that plain parentheses fulfil two functions is not always
       helpful. There are often times when grouping is required without
       capturing. If an opening parenthesis is followed by a question mark and
       a colon, the group does not do any capturing, and is not counted when
       computing the number of any subsequent capture groups. For example, if
       the string "the white queen" is matched against the pattern

       the ((?:red|white) (king|queen))

       the captured substrings are "white queen" and "queen", and are numbered
       1 and 2. The maximum number of capture groups is 65535.

       As a convenient shorthand, if any option settings are required at the
       start of a non-capturing group, the option letters may appear between
       the "?" and the ":". Thus the two patterns

       (?i:saturday|sunday)
       (?:(?i)saturday|sunday)

       match exactly the same set of strings. Because alternative branches are
       tried from left to right, and options are not reset until the end of
       the group is reached, an option setting in one branch does affect
       subsequent branches, so the above patterns match "SUNDAY" as well as
       "Saturday".


DUPLICATE GROUP NUMBERS
       Perl 5.10 introduced a feature whereby each alternative in a group uses
       the same numbers for its capturing parentheses. Such a group starts
       with (?| and is itself a non-capturing group. For example, consider
       this pattern:

       (?|(Sat)ur|(Sun))day

       Because the two alternatives are inside a (?| group, both sets of
       capturing parentheses are numbered one. Thus, when the pattern matches,
       you can look at captured substring number one, whichever alternative
       matched. This construct is useful when you want to capture part, but
       not all, of one of a number of alternatives. Inside a (?| group,
       parentheses are numbered as usual, but the number is reset at the start
       of each branch. The numbers of any capturing parentheses that follow
       the whole group start after the highest number used in any branch. The
       following example is taken from the Perl documentation. The numbers
       underneath show in which buffer the captured content will be stored.

       # before  ---------------branch-reset----------- after
       / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
       # 1            2         2  3        2     3     4

       A backreference to a capture group uses the most recent value that is
       set for the group. The following pattern matches "abcabc" or "defdef":

       /(?|(abc)|(def))\1/

       In contrast, a subroutine call to a capture group always refers to the
       first one in the pattern with the given number. The following pattern
       matches "abcabc" or "defabc":

       /(?|(abc)|(def))(?1)/

       A relative reference such as (?-1) is no different: it is just a
       convenient way of computing an absolute group number.

       If a condition test for a group's having matched refers to a non-unique
       number, the test is true if any group with that number has matched.

       An alternative approach to using this "branch reset" feature is to use
       duplicate named groups, as described in the next section.


NAMED CAPTURE GROUPS
       Identifying capture groups by number is simple, but it can be very hard
       to keep track of the numbers in complicated patterns. Furthermore, if
       an expression is modified, the numbers may change. To help with this
       difficulty, PCRE2 supports the naming of capture groups. This feature
       was not added to Perl until release 5.10. Python had the feature
       earlier, and PCRE1 introduced it at release 4.0, using the Python
       syntax. PCRE2 supports both the Perl and the Python syntax.

       In PCRE2, a capture group can be named in one of three ways:
       (?<name>...) or (?'name'...) as in Perl, or (?P<name>...) as in Python.
       Names may be up to 128 code units long. When PCRE2_UTF is not set, they
       may contain only ASCII alphanumeric characters and underscores, but
       must start with a non-digit. When PCRE2_UTF is set, the syntax of group
       names is extended to allow any Unicode letter or Unicode decimal digit.
       In other words, group names must match one of these patterns:

       ^[_A-Za-z][_A-Za-z0-9]*\z   when PCRE2_UTF is not set
       ^[_\p{L}][_\p{L}\p{Nd}]*\z  when PCRE2_UTF is set

       References to capture groups from other parts of the pattern, such as
       backreferences, recursion, and conditions, can all be made by name as
       well as by number.

       Named capture groups are allocated numbers as well as names, exactly as
       if the names were not present. In both PCRE2 and Perl, capture groups
       are primarily identified by numbers; any names are just aliases for
       these numbers. The PCRE2 API provides function calls for extracting the
       complete name-to-number translation table from a compiled pattern, as
       well as convenience functions for extracting captured substrings by
       name.

       Warning: When more than one capture group has the same number, as
       described in the previous section, a name given to one of them applies
       to all of them. Perl allows identically numbered groups to have
       different names. Consider this pattern, where there are two capture
       groups, both numbered 1:

       (?|(?<AA>aa)|(?<BB>bb))

       Perl allows this, with both names AA and BB as aliases of group 1.
       Thus, after a successful match, both names yield the same value (either
       "aa" or "bb").

       In an attempt to reduce confusion, PCRE2 does not allow the same group
       number to be associated with more than one name. The example above
       provokes a compile-time error. However, there is still scope for
       confusion. Consider this pattern:

       (?|(?<AA>aa)|(bb))

       Although the second group number 1 is not explicitly named, the name AA
       is still an alias for any group 1. Whether the pattern matches "aa" or
       "bb", a reference by name to group AA yields the matched string.

       By default, a name must be unique within a pattern, except that
       duplicate names are permitted for groups with the same number, for
       example:

       (?|(?<AA>aa)|(?<AA>bb))

       The duplicate name constraint can be disabled by setting the
       PCRE2_DUPNAMES option at compile time, or by the use of (?J) within the
       pattern, as described in the section entitled "Internal Option Setting"
       above.

       Duplicate names can be useful for patterns where only one instance of
       the named capture group can match. Suppose you want to match the name
       of a weekday, either as a 3-letter abbreviation or as the full name,
       and in both cases you want to extract the abbreviation. This pattern
       (ignoring the line breaks) does the job:

       (?J)
       (?<DN>Mon|Fri|Sun)(?:day)?|
       (?<DN>Tue)(?:sday)?|
       (?<DN>Wed)(?:nesday)?|
       (?<DN>Thu)(?:rsday)?|
       (?<DN>Sat)(?:urday)?

       There are five capture groups, but only one is ever set after a match.
       The convenience functions for extracting the data by name returns the
       substring for the first (and in this example, the only) group of that
       name that matched. This saves searching to find which numbered group it
       was. (An alternative way of solving this problem is to use a "branch
       reset" group, as described in the previous section.)

       If you make a backreference to a non-unique named group from elsewhere
       in the pattern, the groups to which the name refers are checked in the
       order in which they appear in the overall pattern. The first one that
       is set is used for the reference. For example, this pattern matches
       both "foofoo" and "barbar" but not "foobar" or "barfoo":

       (?J)(?:(?<n>foo)|(?<n>bar))\k<n>

       If you make a subroutine call to a non-unique named group, the one that
       corresponds to the first occurrence of the name is used. In the absence
       of duplicate numbers this is the one with the lowest number.

       If you use a named reference in a condition test (see the section about
       conditions below), either to check whether a capture group has matched,
       or to check for recursion, all groups with the same name are tested. If
       the condition is true for any one of them, the overall condition is
       true. This is the same behaviour as testing by number. For further
       details of the interfaces for handling named capture groups, see the
       pcre2api documentation.


REPETITION
       Repetition is specified by quantifiers, which may follow any one of
       these items:

       a literal data character
       the dot metacharacter
       the \C escape sequence
       the \R escape sequence
       the \X escape sequence
       any escape sequence that matches a single character
       a character class
       a backreference
       a parenthesized group (including lookaround assertions)
       a subroutine call (recursive or otherwise)

       If a quantifier does not follow a repeatable item, an error occurs. The
       general repetition quantifier specifies a minimum and maximum number of
       permitted matches by giving two numbers in curly brackets (braces),
       separated by a comma. The numbers must be less than 65536, and the
       first must be less than or equal to the second. For example,

       z{2,4}

       matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
       special character. If the second number is omitted, but the comma is
       present, there is no upper limit; if the second number and the comma
       are both omitted, the quantifier specifies an exact number of required
       matches. Thus

       [aeiou]{3,}

       matches at least 3 successive vowels, but may match many more, whereas

       \d{8}

       matches exactly 8 digits. If the first number is omitted, the lower
       limit is taken as zero; in this case the upper limit must be present.

       X{,4} is interpreted as X{0,4}

       This is a change in behaviour that happened in Perl 5.34.0 and PCRE2
       10.43. In earlier versions such a sequence was not interpreted as a
       quantifier. Other regular expression engines may behave either way.

       If the characters that follow an opening brace do not match the syntax
       of a quantifier, the brace is taken as a literal character. In
       particular, this means that {,} is a literal string of three
       characters.

       Note that not every opening brace is potentially the start of a
       quantifier because braces are used in other items such as \N{U+345} or
       \k{name}.

       In UTF modes, quantifiers apply to characters rather than to individual
       code units. Thus, for example, \x{100}{2} matches two characters, each
       of which is represented by a two-byte sequence in a UTF-8 string.
       Similarly, \X{3} matches three Unicode extended grapheme clusters, each
       of which may be several code units long (and they may be of different
       lengths).

       The quantifier {0} is permitted, causing the expression to behave as if
       the previous item and the quantifier were not present. This may be
       useful for capture groups that are referenced as subroutines from
       elsewhere in the pattern (but see also the section entitled "Defining
       capture groups for use by reference only" below). Except for
       parenthesized groups, items that have a {0} quantifier are omitted from
       the compiled pattern.

       For convenience, the three most common quantifiers have
       single-character abbreviations:

       *    is equivalent to {0,}
       +    is equivalent to {1,}
       ?    is equivalent to {0,1}

       It is possible to construct infinite loops by following a group that
       can match no characters with a quantifier that has no upper limit, for
       example:

       (a?)*

       Earlier versions of Perl and PCRE1 used to give an error at compile
       time for such patterns. However, because there are cases where this can
       be useful, such patterns are now accepted, but whenever an iteration of
       such a group matches no characters, matching moves on to the next item
       in the pattern instead of repeatedly matching an empty string. This
       does not prevent backtracking into any of the iterations if a
       subsequent item fails to match.

       By default, quantifiers are "greedy", that is, they match as much as
       possible (up to the maximum number of permitted repetitions), without
       causing the rest of the pattern to fail. The classic example of where
       this gives problems is in trying to match comments in C programs. These
       appear between / and / and within the comment, individual * and /
       characters may appear. An attempt to match C comments by applying the
       pattern

       /\*.*\*/

       to the string

       /* first comment */  not comment  /* second comment */

       fails, because it matches the entire string owing to the greediness of
       the .* item. However, if a quantifier is followed by a question mark,
       it ceases to be greedy, and instead matches the minimum number of times
       possible, so the pattern

       /\*.*?\*/

       does the right thing with C comments. The meaning of the various
       quantifiers is not otherwise changed, just the preferred number of
       matches. Do not confuse this use of question mark with its use as a
       quantifier in its own right. Because it has two uses, it can sometimes
       appear doubled, as in

       \d??\d

       which matches one digit by preference, but can match two if that is the
       only way the rest of the pattern matches.

       If the PCRE2_UNGREEDY option is set (an option that is not available in
       Perl), the quantifiers are not greedy by default, but individual ones
       can be made greedy by following them with a question mark. In other
       words, it inverts the default behaviour.

       When a parenthesized group is quantified with a minimum repeat count
       that is greater than 1 or with a limited maximum, more memory is
       required for the compiled pattern, in proportion to the size of the
       minimum or maximum.

       If a pattern starts with .* or .{0,} and the PCRE2_DOTALL option
       (equivalent to Perl's /s) is set, thus allowing the dot to match
       newlines, the pattern is implicitly anchored, because whatever follows
       will be tried against every character position in the subject string,
       so there is no point in retrying the overall match at any position
       after the first. PCRE2 normally treats such a pattern as though it were
       preceded by \A.

       In cases where it is known that the subject string contains no
       newlines, it is worth setting PCRE2_DOTALL in order to obtain this
       optimization, or alternatively, using ^ to indicate anchoring
       explicitly.

       However, there are some cases where the optimization cannot be used.
       When .* is inside capturing parentheses that are the subject of a
       backreference elsewhere in the pattern, a match at the start may fail
       where a later one succeeds. Consider, for example:

       (.*)abc\1

       If the subject is "xyz123abc123" the match point is the fourth
       character. For this reason, such a pattern is not implicitly anchored.

       Another case where implicit anchoring is not applied is when the
       leading .* is inside an atomic group. Once again, a match at the start
       may fail where a later one succeeds. Consider this pattern:

       (?>.*?a)b

       It matches "ab" in the subject "aab". The use of the backtracking
       control verbs (*PRUNE) and (*SKIP) also disable this optimization. To
       do so explicitly, either pass the compile option
       PCRE2_NO_DOTSTAR_ANCHOR, or call pcre2_set_optimize() with a
       PCRE2_DOTSTAR_ANCHOR_OFF directive.

       When a capture group is repeated, the value captured is the substring
       that matched the final iteration. For example, after

       (tweedle[dume]{3}\s*)+

       has matched "tweedledum tweedledee" the value of the captured substring
       is "tweedledee". However, if there are nested capture groups, the
       corresponding captured values may have been set in previous iterations.
       For example, after

       (a|(b))+

       matches "aba" the value of the second captured substring is "b".


ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
       With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
       repetition, failure of what follows normally causes the repeated item
       to be re-evaluated to see if a different number of repeats allows the
       rest of the pattern to match. Sometimes it is useful to prevent this,
       either to change the nature of the match, or to cause it fail earlier
       than it otherwise might, when the author of the pattern knows there is
       no point in carrying on.

       Consider, for example, the pattern \d+foo when applied to the subject
       line

       123456bar

       After matching all 6 digits and then failing to match "foo", the normal
       action of the matcher is to try again with only 5 digits matching the
       \d+ item, and then with 4, and so on, before ultimately failing.
       "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
       the means for specifying that once a group has matched, it is not to be
       re-evaluated in this way.

       If we use atomic grouping for the previous example, the matcher gives
       up immediately on failing to match "foo" the first time. The notation
       is a kind of special parenthesis, starting with (?> as in this example:

       (?>\d+)foo

       Perl 5.28 introduced an experimental alphabetic form starting with (*
       which may be easier to remember:

       (*atomic:\d+)foo

       This kind of parenthesized group "locks up" the part of the pattern it
       contains once it has matched, and a failure further into the pattern is
       prevented from backtracking into it. Backtracking past it to previous
       items, however, works as normal.

       An alternative description is that a group of this type matches exactly
       the string of characters that an identical standalone pattern would
       match, if anchored at the current point in the subject string.

       Atomic groups are not capture groups. Simple cases such as the above
       example can be thought of as a maximizing repeat that must swallow
       everything it can. So, while both \d+ and \d+? are prepared to adjust
       the number of digits they match in order to make the rest of the
       pattern match, (?>\d+) can only match an entire sequence of digits.

       Atomic groups in general can of course contain arbitrarily complicated
       expressions, and can be nested. However, when the contents of an atomic
       group is just a single repeated item, as in the example above, a
       simpler notation, called a "possessive quantifier" can be used. This
       consists of an additional + character following a quantifier. Using
       this notation, the previous example can be rewritten as

       \d++foo

       Note that a possessive quantifier can be used with an entire group, for
       example:

       (abc|xyz){2,3}+

       Possessive quantifiers are always greedy; the setting of the
       PCRE2_UNGREEDY option is ignored. They are a convenient notation for
       the simpler forms of atomic group. However, there is no difference in
       the meaning of a possessive quantifier and the equivalent atomic group,
       though there may be a performance difference; possessive quantifiers
       should be slightly faster.

       The possessive quantifier syntax is an extension to the Perl 5.8
       syntax. Jeffrey Friedl originated the idea (and the name) in the first
       edition of his book. Mike McCloskey liked it, so implemented it when he
       built Sun's Java package, and PCRE1 copied it from there. It found its
       way into Perl at release 5.10.

       PCRE2 has an optimization that automatically "possessifies" certain
       simple pattern constructs. For example, the sequence A+B is treated as
       A++B because there is no point in backtracking into a sequence of A's
       when B must follow. This feature can be disabled by the
       PCRE2_NO_AUTO_POSSESS option, by calling pcre2_set_optimize() with a
       PCRE2_AUTO_POSSESS_OFF directive, or by starting the pattern with
       (*NO_AUTO_POSSESS).

       When a pattern contains an unlimited repeat inside a group that can
       itself be repeated an unlimited number of times, the use of an atomic
       group is the only way to avoid some failing matches taking a very long
       time indeed. The pattern

       (\D+|<\d+>)*[!?]

       matches an unlimited number of substrings that either consist of
       non-digits, or digits enclosed in <>, followed by either ! or ?. When
       it matches, it runs quickly. However, if it is applied to

       aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

       it takes a long time before reporting failure. This is because the
       string can be divided between the internal \D+ repeat and the external
       * repeat in a large number of ways, and all have to be tried. (The
       example uses [!?] rather than a single character at the end, because
       both PCRE2 and Perl have an optimization that allows for fast failure
       when a single character is used. They remember the last single
       character that is required for a match, and fail early if it is not
       present in the string.) If the pattern is changed so that it uses an
       atomic group, like this:

       ((?>\D+)|<\d+>)*[!?]

       sequences of non-digits cannot be broken, and failure happens quickly.


BACKREFERENCES
       Outside a character class, a backslash followed by a digit greater than
       0 (and possibly further digits) is a backreference to a capture group
       earlier (that is, to its left) in the pattern, provided there have been
       that many previous capture groups.

       However, if the decimal number following the backslash is less than 8,
       it is always taken as a backreference, and causes an error only if
       there are not that many capture groups in the entire pattern. In other
       words, the group that is referenced need not be to the left of the
       reference for numbers less than 8. A "forward backreference" of this
       type can make sense when a repetition is involved and the group to the
       right has participated in an earlier iteration.

       It is not possible to have a numerical "forward backreference" to a
       group whose number is 8 or more using this syntax because a sequence
       such as \50 is interpreted as a character defined in octal. See the
       subsection entitled "Non-printing characters" above for further details
       of the handling of digits following a backslash. Other forms of
       backreferencing do not suffer from this restriction. In particular,
       there is no problem when named capture groups are used (see below).

       Another way of avoiding the ambiguity inherent in the use of digits
       following a backslash is to use the \g escape sequence. This escape
       must be followed by a signed or unsigned number, optionally enclosed in
       braces. These examples are all identical:

       (ring), \1
       (ring), \g1
       (ring), \g{1}

       An unsigned number specifies an absolute reference without the
       ambiguity that is present in the older syntax. It is also useful when
       literal digits follow the reference. A signed number is a relative
       reference. Consider this example:

       (abc(def)ghi)\g{-1}

       The sequence \g{-1} is a reference to the capture group whose number is
       one less than the number of the next group to be started, so in this
       example (where the next group would be numbered 3) is it equivalent to
       \2, and \g{-2} would be equivalent to \1. Note that if this construct
       is inside a capture group, that group is included in the count, so in
       this example \g{-2} also refers to group 1:

       (A)(\g{-2}B)

       The use of relative references can be helpful in long patterns, and
       also in patterns that are created by joining together fragments that
       contain references within themselves.

       The sequence \g{+1} is a reference to the next capture group that is
       started after this item, and \g{+2} refers to the one after that, and
       so on. This kind of forward reference can be useful in patterns that
       repeat. Perl does not support the use of + in this way.

       A backreference matches whatever actually most recently matched the
       capture group in the current subject string, rather than anything at
       all that matches the group (see "Groups as subroutines" below for a way
       of doing that). So the pattern

       (sens|respons)e and \1ibility

       matches "sense and sensibility" and "response and responsibility", but
       not "sense and responsibility". If caseful matching is in force at the
       time of the backreference, the case of letters is relevant. For
       example,

       ((?i)rah)\s+\1

       matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
       original capture group is matched caselessly.

       There are several different ways of writing backreferences to named
       capture groups. The .NET syntax is \k{name}, the Python syntax is
       (?=name), and the original Perl syntax is \k<name> or \k'name'. All of
       these are now supported by both Perl and PCRE2. Perl 5.10's unified
       backreference syntax, in which \g can be used for both numeric and
       named references, is also supported by PCRE2. We could rewrite the
       above example in any of the following ways:

       (?<p1>(?i)rah)\s+\k<p1>
       (?'p1'(?i)rah)\s+\k{p1}
       (?P<p1>(?i)rah)\s+(?P=p1)
       (?<p1>(?i)rah)\s+\g{p1}

       A capture group that is referenced by name may appear in the pattern
       before or after the reference.

       There may be more than one backreference to the same group. If a group
       has not actually been used in a particular match, backreferences to it
       always fail by default. For example, the pattern

       (a|(bc))\2

       always fails if it starts to match "a" rather than "bc". However, if
       the PCRE2_MATCH_UNSET_BACKREF option is set at compile time, a
       backreference to an unset value matches an empty string.

       Because there may be many capture groups in a pattern, all digits
       following a backslash are taken as part of a potential backreference
       number. If the pattern continues with a digit character, some delimiter
       must be used to terminate the backreference. If the PCRE2_EXTENDED or
       PCRE2_EXTENDED_MORE option is set, this can be white space. Otherwise,
       the \g{} syntax or an empty comment (see "Comments" below) can be used.

   Recursive backreferences
       A backreference that occurs inside the group to which it refers fails
       when the group is first used, so, for example, (a\1) never matches.
       However, such references can be useful inside repeated groups. For
       example, the pattern

       (a|b\1)+

       matches any number of "a"s and also "aba", "ababbaa" etc. At each
       iteration of the group, the backreference matches the character string
       corresponding to the previous iteration. In order for this to work, the
       pattern must be such that the first iteration does not need to match
       the backreference. This can be done using alternation, as in the
       example above, or by a quantifier with a minimum of zero.

       For versions of PCRE2 less than 10.25, backreferences of this type used
       to cause the group that they reference to be treated as an atomic
       group. This restriction no longer applies, and backtracking into such
       groups can occur as normal.


ASSERTIONS
       An assertion is a test that does not consume any characters. The test
       must succeed for the match to continue. The simple assertions coded as
       \b, \B, \A, \G, \Z, \z, ^ and $ are described above.

       More complicated assertions are coded as parenthesized groups. If
       matching such a group succeeds, matching continues after it, but with
       the matching position in the subject string reset to what it was before
       the assertion was processed.

       A special kind of assertion, called a "scan substring" assertion,
       matches a subpattern against a previously captured substring. This is
       described in the section entitled "Scan substring assertions" below. It
       is a PCRE2 extension, not compatible with Perl.

       The other goup-based assertions are of two kinds: those that look ahead
       of the current position in the subject string, and those that look
       behind it, and in each case an assertion may be positive (must match
       for the assertion to be true) or negative (must not match for the
       assertion to be true).

       The Perl-compatible lookaround assertions are atomic. If an assertion
       is true, but there is a subsequent matching failure, there is no
       backtracking into the assertion. However, there are some cases where
       non-atomic assertions can be useful. PCRE2 has some support for these,
       described in the section entitled "Non-atomic assertions" below, but
       they are not Perl-compatible.

       A lookaround assertion may appear as the condition in a conditional
       group (see below). In this case, the result of matching the assertion
       determines which branch of the condition is followed.

       Assertion groups are not capture groups. If an assertion contains
       capture groups within it, these are counted for the purposes of
       numbering the capture groups in the whole pattern. Within each branch
       of an assertion, locally captured substrings may be referenced in the
       usual way. For example, a sequence such as (.)\g{-1} can be used to
       check that two adjacent characters are the same.

       When a branch within an assertion fails to match, any substrings that
       were captured are discarded (as happens with any pattern branch that
       fails to match). A negative assertion is true only when all its
       branches fail to match; this means that no captured substrings are ever
       retained after a successful negative assertion. When an assertion
       contains a matching branch, what happens depends on the type of
       assertion.

       For a positive assertion, internally captured substrings in the
       successful branch are retained, and matching continues with the next
       pattern item after the assertion. For a negative assertion, a matching
       branch means that the assertion is not true. If such an assertion is
       being used as a condition in a conditional group (see below), captured
       substrings are retained, because matching continues with the "no"
       branch of the condition. For other failing negative assertions, control
       passes to the previous backtracking point, thus discarding any captured
       strings within the assertion.

       Most assertion groups may be repeated; though it makes no sense to
       assert the same thing several times, the side effect of capturing in
       positive assertions may occasionally be useful. However, an assertion
       that forms the condition for a conditional group may not be quantified.
       PCRE2 used to restrict the repetition of assertions, but from release
       10.35 the only restriction is that an unlimited maximum repetition is
       changed to be one more than the minimum. For example, {3,} is treated
       as {3,4}.

   Alphabetic assertion names
       Traditionally, symbolic sequences such as (?= and (?<= have been used
       to specify lookaround assertions. Perl 5.28 introduced some
       experimental alphabetic alternatives which might be easier to remember.
       They all start with (* instead of (? and must be written using lower
       case letters. PCRE2 supports the following synonyms:

       (*positive_lookahead:  or (*pla: is the same as (?=
       (*negative_lookahead:  or (*nla: is the same as (?!
       (*positive_lookbehind: or (*plb: is the same as (?<=
       (*negative_lookbehind: or (*nlb: is the same as (?<!

       For example, (*pla:foo) is the same assertion as (?=foo). In the
       following sections, the various assertions are described using the
       original symbolic forms.

   Lookahead assertions
       Lookahead assertions start with (?= for positive assertions and (?! for
       negative assertions. For example,

       \w+(?=;)

       matches a word followed by a semicolon, but does not include the
       semicolon in the match, and

       foo(?!bar)

       matches any occurrence of "foo" that is not followed by "bar". Note
       that the apparently similar pattern

       (?!foo)bar

       does not find an occurrence of "bar" that is preceded by something
       other than "foo"; it finds any occurrence of "bar" whatsoever, because
       the assertion (?!foo) is always true when the next three characters are
       "bar". A lookbehind assertion is needed to achieve the other effect.

       If you want to force a matching failure at some point in a pattern, the
       most convenient way to do it is with (?!) because an empty string
       always matches, so an assertion that requires there not to be an empty
       string must always fail. The backtracking control verb (*FAIL) or (*F)
       is a synonym for (?!).

   Lookbehind assertions
       Lookbehind assertions start with (?<= for positive assertions and (?<!
       for negative assertions. For example,

       (?<!foo)bar

       does find an occurrence of "bar" that is not preceded by "foo". The
       contents of a lookbehind assertion are restricted such that there must
       be a known maximum to the lengths of all the strings it matches. There
       are two cases:

       If every top-level alternative matches a fixed length, for example

       (?<=colour|color)

       there is a limit of 65535 characters to the lengths, which do not have
       to be the same, as this example demonstrates. This is the only kind of
       lookbehind supported by PCRE2 versions earlier than 10.43 and by the
       alternative matching function pcre2_dfa_match().

       In PCRE2 10.43 and later, pcre2_match() supports lookbehind assertions
       in which one or more top-level alternatives can match more than one
       string length, for example

       (?<=colou?r)

       The maximum matching length for any branch of the lookbehind is limited
       to a value set by the calling program (default 255 characters).
       Unlimited repetition (for example \d*) is not supported. In some cases,
       the escape sequence \K (see above) can be used instead of a lookbehind
       assertion at the start of a pattern to get round the length limit
       restriction.

       In UTF-8 and UTF-16 modes, PCRE2 does not allow the \C escape (which
       matches a single code unit even in a UTF mode) to appear in lookbehind
       assertions, because it makes it impossible to calculate the length of
       the lookbehind. The \X and \R escapes, which can match different
       numbers of code units, are never permitted in lookbehinds.

       "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
       lookbehinds, as long as the called capture group matches a
       limited-length string. However, recursion, that is, a "subroutine" call
       into a group that is already active, is not supported.

       PCRE2 supports backreferences in lookbehinds, but only if certain
       conditions are met. The PCRE2_MATCH_UNSET_BACKREF option must not be
       set, there must be no use of (?| in the pattern (it creates duplicate
       group numbers), and if the backreference is by name, the name must be
       unique. Of course, the referenced group must itself match a limited
       length substring. The following pattern matches words containing at
       least two characters that begin and end with the same character:

        \b(\w)\w++(?<=\1)

       Possessive quantifiers can be used in conjunction with lookbehind
       assertions to specify efficient matching at the end of subject strings.
       Consider a simple pattern such as

       abcd$

       when applied to a long string that does not match. Because matching
       proceeds from left to right, PCRE2 will look for each "a" in the
       subject and then see if what follows matches the rest of the pattern.
       If the pattern is specified as

       ^.*abcd$

       the initial .* matches the entire string at first, but when this fails
       (because there is no following "a"), it backtracks to match all but the
       last character, then all but the last two characters, and so on. Once
       again the search for "a" covers the entire string, from right to left,
       so we are no better off. However, if the pattern is written as

       ^.*+(?<=abcd)

       there can be no backtracking for the .*+ item because of the possessive
       quantifier; it can match only the entire string. The subsequent
       lookbehind assertion does a single test on the last four characters. If
       it fails, the match fails immediately. For long strings, this approach
       makes a significant difference to the processing time.

   Using multiple assertions
       Several assertions (of any sort) may occur in succession. For example,

       (?<=\d{3})(?<!999)foo

       matches "foo" preceded by three digits that are not "999". Notice that
       each of the assertions is applied independently at the same point in
       the subject string. First there is a check that the previous three
       characters are all digits, and then there is a check that the same
       three characters are not "999". This pattern does not match "foo"
       preceded by six characters, the first of which are digits and the last
       three of which are not "999". For example, it doesn't match
       "123abcfoo". A pattern to do that is

       (?<=\d{3}...)(?<!999)foo

       This time the first assertion looks at the preceding six characters,
       checking that the first three are digits, and then the second assertion
       checks that the preceding three characters are not "999".

       Assertions can be nested in any combination. For example,

       (?<=(?<!foo)bar)baz

       matches an occurrence of "baz" that is preceded by "bar" which in turn
       is not preceded by "foo", while

       (?<=\d{3}(?!999)...)foo

       is another pattern that matches "foo" preceded by three digits and any
       three characters that are not "999".


NON-ATOMIC ASSERTIONS
       Traditional lookaround assertions are atomic. That is, if an assertion
       is true, but there is a subsequent matching failure, there is no
       backtracking into the assertion. However, there are some cases where
       non-atomic positive assertions can be useful. PCRE2 provides these
       using the following syntax:

       (*non_atomic_positive_lookahead:  or (*napla: or (?*
       (*non_atomic_positive_lookbehind: or (*naplb: or (?<*

       Consider the problem of finding the right-most word in a string that
       also appears earlier in the string, that is, it must appear at least
       twice in total. This pattern returns the required result as captured
       substring 1:

       ^(?x)(*napla: .* \b(\w++)) (?> .*? \b\1\b ){2}

       For a subject such as "word1 word2 word3 word2 word3 word4" the result
       is "word3". How does it work? At the start, ^(?x) anchors the pattern
       and sets the "x" option, which causes white space (introduced for
       readability) to be ignored. Inside the assertion, the greedy .* at
       first consumes the entire string, but then has to backtrack until the
       rest of the assertion can match a word, which is captured by group 1.
       In other words, when the assertion first succeeds, it captures the
       right-most word in the string.

       The current matching point is then reset to the start of the subject,
       and the rest of the pattern match checks for two occurrences of the
       captured word, using an ungreedy .*? to scan from the left. If this
       succeeds, we are done, but if the last word in the string does not
       occur twice, this part of the pattern fails. If a traditional atomic
       lookahead (?= or (*pla: had been used, the assertion could not be
       re-entered, and the whole match would fail. The pattern would succeed
       only if the very last word in the subject was found twice.

       Using a non-atomic lookahead, however, means that when the last word
       does not occur twice in the string, the lookahead can backtrack and
       find the second-last word, and so on, until either the match succeeds,
       or all words have been tested.

       Two conditions must be met for a non-atomic assertion to be useful: the
       contents of one or more capturing groups must change after a backtrack
       into the assertion, and there must be a backreference to a changed
       group later in the pattern. If this is not the case, the rest of the
       pattern match fails exactly as before because nothing has changed, so
       using a non-atomic assertion just wastes resources.

       There is one exception to backtracking into a non-atomic assertion. If
       an (*ACCEPT) control verb is triggered, the assertion succeeds
       atomically. That is, a subsequent match failure cannot backtrack into
       the assertion.

       Non-atomic assertions are not supported by the alternative matching
       function pcre2_dfa_match(). They are supported by JIT, but only if they
       do not contain any control verbs such as (*ACCEPT). (This may change in
       future). Note that assertions that appear as conditions for conditional
       groups (see below) must be atomic.


SCAN SUBSTRING ASSERTIONS
       A special kind of assertion, not compatible with Perl, makes it
       possible to check the contents of a captured substring by matching it
       with a subpattern. Because this involves capturing, this feature is not
       supported by pcre2_dfa_match().

       A scan substring assertion starts with the sequence (*scan_substring:
       or (*scs: which is followed by a list of substring numbers (absolute or
       relative) and/or substring names enclosed in single quotes or angle
       brackets, all within parentheses. The rest of the item is the
       subpattern that is applied to the substring, as shown in these
       examples:

       (*scan_substring:(1)...)
       (*scs:(-2)...)
       (*scs:('AB')...)
       (*scs:(1,'AB',-2)...)

       The list of groups is checked in the order they are given, and it is
       the contents of the first one that is found to be set that are scanned.
       When PCRE2_DUPNAMES is set and there are ambiguous group names, all
       groups with the same name are checked in numerical order. A scan
       substring assertion fails if none of the groups it references have been
       set.

       The pattern match on the substring is always anchored, that is, it must
       match from the start of the substring. There is no "bumpalong" if it
       does not match at the start. The end of the subject is temporarily
       reset to be the end of the substring, so \Z, \z, and $ will match
       there. However, the start of the subject is not reset. This means that
       ^ matches only if the substring is actually at the start of the main
       subject, but it also means that lookbehind assertions into what
       precedes the substring are possible.

       Here is a very simple example: find a word that contains the rare (in
       English) sequence of letters "rh" not at the start:

       \b(\w++)(*scs:(1).+rh)

       The first group captures a word which is then scanned by the second
       group. This example does not actually need this heavyweight feature;
       the same match can be achieved with:

       \b\w+?rh\w*\b

       When things are more complicated, however, scanning a captured
       substring can be a useful way to describe the required match. For
       exmple, there is a rather complicated pattern in the PCRE2 test data
       that checks an entire subject string for a palindrome, that is, the
       sequence of letters is the same in both directions. Suppose you want to
       search for individual words of two or more characters such as "level"
       that are palindromes:

       (\b\w{2,}+\b)(*scs:(1)...palindrome-matching-pattern...)

       Within a substring scanning subpattern, references to other groups work
       as normal. Capturing groups may appear, and will retain their values
       during ongoing matching if the assertion succeeds.


SCRIPT RUNS
       In concept, a script run is a sequence of characters that are all from
       the same Unicode script such as Latin or Greek. However, because some
       scripts are commonly used together, and because some diacritical and
       other marks are used with multiple scripts, it is not that simple.
       There is a full description of the rules that PCRE2 uses in the section
       entitled "Script Runs" in the pcre2unicode documentation.

       If part of a pattern is enclosed between (*script_run: or (*sr: and a
       closing parenthesis, it fails if the sequence of characters that it
       matches are not a script run. After a failure, normal backtracking
       occurs. Script runs can be used to detect spoofing attacks using
       characters that look the same, but are from different scripts. The
       string "paypal.com" is an infamous example, where the letters could be
       a mixture of Latin and Cyrillic. This pattern ensures that the matched
       characters in a sequence of non-spaces that follow white space are a
       script run:

       \s+(*sr:\S+)

       To be sure that they are all from the Latin script (for example), a
       lookahead can be used:

       \s+(?=\p{Latin})(*sr:\S+)

       This works as long as the first character is expected to be a character
       in that script, and not (for example) punctuation, which is allowed
       with any script. If this is not the case, a more creative lookahead is
       needed. For example, if digits, underscore, and dots are permitted at
       the start:

       \s+(?=[0-9_.]*\p{Latin})(*sr:\S+)

       In many cases, backtracking into a script run pattern fragment is not
       desirable. The script run can employ an atomic group to prevent this.
       Because this is a common requirement, a shorthand notation is provided
       by (*atomic_script_run: or (*asr:

       (*asr:...) is the same as (*sr:(?>...))

       Note that the atomic group is inside the script run. Putting it outside
       would not prevent backtracking into the script run pattern.

       Support for script runs is not available if PCRE2 is compiled without
       Unicode support. A compile-time error is given if any of the above
       constructs is encountered. Script runs are not supported by the
       alternate matching function, pcre2_dfa_match() because they use the
       same mechanism as capturing parentheses.

       Warning: The (*ACCEPT) control verb (see below) should not be used
       within a script run group, because it causes an immediate exit from the
       group, bypassing the script run checking.


CONDITIONAL GROUPS
       It is possible to cause the matching process to obey a pattern fragment
       conditionally or to choose between two alternative fragments, depending
       on the result of an assertion, or whether a specific capture group has
       already been matched. The two possible forms of conditional group are:

       (?(condition)yes-pattern)
       (?(condition)yes-pattern|no-pattern)

       If the condition is satisfied, the yes-pattern is used; otherwise the
       no-pattern (if present) is used. An absent no-pattern is equivalent to
       an empty string (it always matches). If there are more than two
       alternatives in the group, a compile-time error occurs. Each of the two
       alternatives may itself contain nested groups of any form, including
       conditional groups; the restriction to two alternatives applies only at
       the level of the condition itself. This pattern fragment is an example
       where the alternatives are complex:

       (?(1) (A|B|C) | (D | (?(2)E|F) | E) )

       There are five kinds of condition: references to capture groups,
       references to recursion, two pseudo-conditions called DEFINE and
       VERSION, and assertions.

   Checking for a used capture group by number
       If the text between the parentheses consists of a sequence of digits,
       the condition is true if a capture group of that number has previously
       matched. If there is more than one capture group with the same number
       (see the earlier section about duplicate group numbers), the condition
       is true if any of them have matched. An alternative notation, which is
       a PCRE2 extension, not supported by Perl, is to precede the digits with
       a plus or minus sign. In this case, the group number is relative rather
       than absolute. The most recently opened capture group (which could be
       enclosing this condition) can be referenced by (?(-1), the next most
       recent by (?(-2), and so on. Inside loops it can also make sense to
       refer to subsequent groups. The next capture group to be opened can be
       referenced as (?(+1), and so on. The value zero in any of these forms
       is not used; it provokes a compile-time error.

       Consider the following pattern, which contains non-significant white
       space to make it more readable (assume the PCRE2_EXTENDED option) and
       to divide it into three parts for ease of discussion:

       ( \( )?    [^()]+    (?(1) \) )

       The first part matches an optional opening parenthesis, and if that
       character is present, sets it as the first captured substring. The
       second part matches one or more characters that are not parentheses.
       The third part is a conditional group that tests whether or not the
       first capture group matched. If it did, that is, if subject started
       with an opening parenthesis, the condition is true, and so the
       yes-pattern is executed and a closing parenthesis is required.
       Otherwise, since no-pattern is not present, the conditional group
       matches nothing. In other words, this pattern matches a sequence of
       non-parentheses, optionally enclosed in parentheses.

       If you were embedding this pattern in a larger one, you could use a
       relative reference:

       ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...

       This makes the fragment independent of the parentheses in the larger
       pattern.

   Checking for a used capture group by name
       Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
       used capture group by name. For compatibility with earlier versions of
       PCRE1, which had this facility before Perl, the syntax (?(name)...) is
       also recognized. Note, however, that undelimited names consisting of
       the letter R followed by digits are ambiguous (see the following
       section). Rewriting the above example to use a named group gives this:

       (?<OPEN> \( )?    [^()]+    (?(<OPEN>) \) )

       If the name used in a condition of this kind is a duplicate, the test
       is applied to all groups of the same name, and is true if any one of
       them has matched.

   Checking for pattern recursion
       "Recursion" in this sense refers to any subroutine-like call from one
       part of the pattern to another, whether or not it is actually
       recursive. See the sections entitled "Recursive patterns" and "Groups
       as subroutines" below for details of recursion and subroutine calls.

       If a condition is the string (R), and there is no capture group with
       the name R, the condition is true if matching is currently in a
       recursion or subroutine call to the whole pattern or any capture group.
       If digits follow the letter R, and there is no group with that name,
       the condition is true if the most recent call is into a group with the
       given number, which must exist somewhere in the overall pattern. This
       is a contrived example that is equivalent to a+b:

       ((?(R1)a+|(?1)b))

       However, in both cases, if there is a capture group with a matching
       name, the condition tests for its being set, as described in the
       section above, instead of testing for recursion. For example, creating
       a group with the name R1 by adding (?<R1>) to the above pattern
       completely changes its meaning.

       If a name preceded by ampersand follows the letter R, for example:

       (?(R&name)...)

       the condition is true if the most recent recursion is into a group of
       that name (which must exist within the pattern).

       This condition does not check the entire recursion stack. It tests only
       the current level. If the name used in a condition of this kind is a
       duplicate, the test is applied to all groups of the same name, and is
       true if any one of them is the most recent recursion.

       At "top level", all these recursion test conditions are false.

   Defining capture groups for use by reference only
       If the condition is the string (DEFINE), the condition is always false,
       even if there is a group with the name DEFINE. In this case, there may
       be only one alternative in the rest of the conditional group. It is
       always skipped if control reaches this point in the pattern; the idea
       of DEFINE is that it can be used to define subroutines that can be
       referenced from elsewhere. (The use of subroutines is described below.)
       For example, a pattern to match an IPv4 address such as
       "192.168.23.245" could be written like this (ignore white space and
       line breaks):

       (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
       \b (?&byte) (\.(?&byte)){3} \b

       The first part of the pattern is a DEFINE group inside which another
       group named "byte" is defined. This matches an individual component of
       an IPv4 address (a number less than 256). When matching takes place,
       this part of the pattern is skipped because DEFINE acts like a false
       condition. The rest of the pattern uses references to the named group
       to match the four dot-separated components of an IPv4 address,
       insisting on a word boundary at each end.

   Checking the PCRE2 version
       Programs that link with a PCRE2 library can check the version by
       calling pcre2_config() with appropriate arguments. Users of
       applications that do not have access to the underlying code cannot do
       this. A special "condition" called VERSION exists to allow such users
       to discover which version of PCRE2 they are dealing with by using this
       condition to match a string such as "yesno". VERSION must be followed
       either by "=" or ">=" and a version number. For example:

       (?(VERSION>=10.4)yes|no)

       This pattern matches "yes" if the PCRE2 version is greater or equal to
       10.4, or "no" otherwise. The fractional part of the version number
       could be ommited.

   Assertion conditions
       If the condition is not in any of the above formats, it must be a
       parenthesized assertion. This may be a positive or negative lookahead
       or lookbehind assertion. However, it must be a traditional atomic
       assertion, not one of the non-atomic assertions.

       Consider this pattern, again containing non-significant white space,
       and with the two alternatives on the second line:

       (?(?=[^a-z]*[a-z])
       \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

       The condition is a positive lookahead assertion that matches an
       optional sequence of non-letters followed by a letter. In other words,
       it tests for the presence of at least one letter in the subject. If a
       letter is found, the subject is matched against the first alternative;
       otherwise it is matched against the second. This pattern matches
       strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
       letters and dd are digits.

       When an assertion that is a condition contains capture groups, any
       capturing that occurs in a matching branch is retained afterwards, for
       both positive and negative assertions, because matching always
       continues after the assertion, whether it succeeds or fails. (Compare
       non-conditional assertions, for which captures are retained only for
       positive assertions that succeed.)


COMMENTS
       There are two ways of including comments in patterns that are processed
       by PCRE2. In both cases, the start of the comment must not be in a
       character class, nor in the middle of any other sequence of related
       characters such as (?: or a group name or number or a Unicode property
       name. The characters that make up a comment play no part in the pattern
       matching.

       The sequence (?# marks the start of a comment that continues up to the
       next closing parenthesis. Nested parentheses are not permitted. If the
       PCRE2_EXTENDED or PCRE2_EXTENDED_MORE option is set, an unescaped #
       character also introduces a comment, which in this case continues to
       immediately after the next newline character or character sequence in
       the pattern. Which characters are interpreted as newlines is controlled
       by an option passed to the compiling function or by a special sequence
       at the start of the pattern, as described in the section entitled
       "Newline conventions" above. Note that the end of this type of comment
       is a literal newline sequence in the pattern; escape sequences that
       happen to represent a newline do not count. For example, consider this
       pattern when PCRE2_EXTENDED is set, and the default newline convention
       (a single linefeed character) is in force:

       abc #comment \n still comment

       On encountering the # character, pcre2_compile() skips along, looking
       for a newline in the pattern. The sequence \n is still literal at this
       stage, so it does not terminate the comment. Only an actual character
       with the code value 0x0a (the default newline) does so.


RECURSIVE PATTERNS
       Consider the problem of matching a string in parentheses, allowing for
       unlimited nested parentheses. Without the use of recursion, the best
       that can be done is to use a pattern that matches up to some fixed
       depth of nesting. It is not possible to handle an arbitrary nesting
       depth.

       For some time, Perl has provided a facility that allows regular
       expressions to recurse (amongst other things). It does this by
       interpolating Perl code in the expression at run time, and the code can
       refer to the expression itself. A Perl pattern using code interpolation
       to solve the parentheses problem can be created like this:

       $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;

       The (?p{...}) item interpolates Perl code at run time, and in this case
       refers recursively to the pattern in which it appears.

       Obviously, PCRE2 cannot support the interpolation of Perl code.
       Instead, it supports special syntax for recursion of the entire
       pattern, and also for individual capture group recursion. After its
       introduction in PCRE1 and Python, this kind of recursion was
       subsequently introduced into Perl at release 5.10.

       A special item that consists of (? followed by a number greater than
       zero and a closing parenthesis is a recursive subroutine call of the
       capture group of the given number, provided that it occurs inside that
       group. (If not, it is a non-recursive subroutine call, which is
       described in the next section.) The special item (?R) or (?0) is a
       recursive call of the entire regular expression.

       This PCRE2 pattern solves the nested parentheses problem (assume the
       PCRE2_EXTENDED option is set so that white space is ignored):

       \( ( [^()]++ | (?R) )* \)

       First it matches an opening parenthesis. Then it matches any number of
       substrings which can either be a sequence of non-parentheses, or a
       recursive match of the pattern itself (that is, a correctly
       parenthesized substring). Finally there is a closing parenthesis. Note
       the use of a possessive quantifier to avoid backtracking into sequences
       of non-parentheses.

       If this were part of a larger pattern, you would not want to recurse
       the entire pattern, so instead you could use this:

       ( \( ( [^()]++ | (?1) )* \) )

       We have put the pattern into parentheses, and caused the recursion to
       refer to them instead of the whole pattern.

       In a larger pattern, keeping track of parenthesis numbers can be
       tricky. This is made easier by the use of relative references. Instead
       of (?1) in the pattern above you can write (?-2) to refer to the second
       most recently opened parentheses preceding the recursion. In other
       words, a negative number counts capturing parentheses leftwards from
       the point at which it is encountered.

       Be aware however, that if duplicate capture group numbers are in use,
       relative references refer to the earliest group with the appropriate
       number. Consider, for example:

       (?|(a)|(b)) (c) (?-2)

       The first two capture groups (a) and (b) are both numbered 1, and group
       (c) is number 2. When the reference (?-2) is encountered, the second
       most recently opened parentheses has the number 1, but it is the first
       such group (the (a) group) to which the recursion refers. This would be
       the same if an absolute reference (?1) was used. In other words,
       relative references are just a shorthand for computing a group number.

       It is also possible to refer to subsequent capture groups, by writing
       references such as (?+2). However, these cannot be recursive because
       the reference is not inside the parentheses that are referenced. They
       are always non-recursive subroutine calls, as described in the next
       section.

       An alternative approach is to use named parentheses. The Perl syntax
       for this is (?&name); PCRE1's earlier syntax (?P>name) is also
       supported. We could rewrite the above example as follows:

       (?<pn> \( ( [^()]++ | (?&pn) )* \) )

       If there is more than one group with the same name, the earliest one is
       used.

       The example pattern that we have been looking at contains nested
       unlimited repeats, and so the use of a possessive quantifier for
       matching strings of non-parentheses is important when applying the
       pattern to strings that do not match. For example, when this pattern is
       applied to

       (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

       it yields "no match" quickly. However, if a possessive quantifier is
       not used, the match runs for a very long time indeed because there are
       so many different ways the + and * repeats can carve up the subject,
       and all have to be tested before failure can be reported.

       At the end of a match, the values of capturing parentheses are those
       from the outermost level. If you want to obtain intermediate values, a
       callout function can be used (see below and the pcre2callout
       documentation). If the pattern above is matched against

       (ab(cd)ef)

       the value for the inner capturing parentheses (numbered 2) is "ef",
       which is the last value taken on at the top level. If a capture group
       is not matched at the top level, its final captured value is unset,
       even if it was (temporarily) set at a deeper level during the matching
       process.

       Do not confuse the (?R) item with the condition (R), which tests for
       recursion. Consider this pattern, which matches text in angle brackets,
       allowing for arbitrary nesting. Only digits are allowed in nested
       brackets (that is, when recursing), whereas any characters are
       permitted at the outer level.

       < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >

       In this pattern, (?(R) is the start of a conditional group, with two
       different alternatives for the recursive and non-recursive cases. The
       (?R) item is the actual recursive call.

   Differences in recursion processing between PCRE2 and Perl
       Some former differences between PCRE2 and Perl no longer exist.

       Before release 10.30, recursion processing in PCRE2 differed from Perl
       in that a recursive subroutine call was always treated as an atomic
       group. That is, once it had matched some of the subject string, it was
       never re-entered, even if it contained untried alternatives and there
       was a subsequent matching failure. (Historical note: PCRE implemented
       recursion before Perl did.)

       Starting with release 10.30, recursive subroutine calls are no longer
       treated as atomic. That is, they can be re-entered to try unused
       alternatives if there is a matching failure later in the pattern. This
       is now compatible with the way Perl works. If you want a subroutine
       call to be atomic, you must explicitly enclose it in an atomic group.

       Supporting backtracking into recursions simplifies certain types of
       recursive pattern. For example, this pattern matches palindromic
       strings:

       ^((.)(?1)\2|.?)$

       The second branch in the group matches a single central character in
       the palindrome when there are an odd number of characters, or nothing
       when there are an even number of characters, but in order to work it
       has to be able to try the second case when the rest of the pattern
       match fails. If you want to match typical palindromic phrases, the
       pattern has to ignore all non-word characters, which can be done like
       this:

       ^\W*+((.)\W*+(?1)\W*+\2|\W*+.?)\W*+$

       If run with the PCRE2_CASELESS option, this pattern matches phrases
       such as "A man, a plan, a canal: Panama!". Note the use of the
       possessive quantifier *+ to avoid backtracking into sequences of
       non-word characters. Without this, PCRE2 takes a great deal longer (ten
       times or more) to match typical phrases, and Perl takes so long that
       you think it has gone into a loop.

       Another way in which PCRE2 and Perl used to differ in their recursion
       processing is in the handling of captured values. Formerly in Perl,
       when a group was called recursively or as a subroutine (see the next
       section), it had no access to any values that were captured outside the
       recursion, whereas in PCRE2 these values can be referenced. Consider
       this pattern:

       ^(.)(\1|a(?2))

       This pattern matches "bab". The first capturing parentheses match "b",
       then in the second group, when the backreference \1 fails to match "b",
       the second alternative matches "a" and then recurses. In the recursion,
       \1 does now match "b" and so the whole match succeeds. This match used
       to fail in Perl, but in later versions (I tried 5.024) it now works.

   Groups as subroutines
       If the syntax for a recursive group call (either by number or by name)
       is used outside the parentheses to which it refers, it operates a bit
       like a subroutine in a programming language. More accurately, PCRE2
       treats the referenced group as an independent subpattern which it tries
       to match at the current matching position. The called group may be
       defined before or after the reference. A numbered reference can be
       absolute or relative, as in these examples:

       (...(absolute)...)...(?2)...
       (...(relative)...)...(?-1)...
       (...(?+1)...(relative)...

       An earlier example pointed out that the pattern

       (sens|respons)e and \1ibility

       matches "sense and sensibility" and "response and responsibility", but
       not "sense and responsibility". If instead the pattern

       (sens|respons)e and (?1)ibility

       is used, it does match "sense and responsibility" as well as the other
       two strings. Another example is given in the discussion of DEFINE
       above.

       Like recursions, subroutine calls used to be treated as atomic, but
       this changed at PCRE2 release 10.30, so backtracking into subroutine
       calls can now occur. However, any capturing parentheses that are set
       during the subroutine call revert to their previous values afterwards.

       Processing options such as case-independence are fixed when a group is
       defined, so if it is used as a subroutine, such options cannot be
       changed for different calls. For example, consider this pattern:

       (abc)(?i:(?-1))

       It matches "abcabc". It does not match "abcABC" because the change of
       processing option does not affect the called group.

       The behaviour of backtracking control verbs in groups when called as
       subroutines is described in the section entitled "Backtracking verbs in
       subroutines" below.

   Recursion and subroutines with returned capture groups
       Since PCRE2 10.47, recursion and subroutine calls may also specify a
       list of capture groups to return. This is a PCRE2 syntax extension not
       supported by Perl. The pattern matching recurses into the referenced
       expression as described above, however, when the recursion returns to
       the calling expression the subgroups captured during the recursion can
       be retained when the calling expression's context is restored.

       When used as a subroutine, this allows the subroutine's capture groups
       to be used as return values.

       Only the specific capture groups listed by the caller will be retained,
       using the following syntax:

       (?R(grouplist))       recurse whole pattern, returning capture groups
       (?n(grouplist))       )
       (?+n(grouplist))      )
       (?-n(grouplist))      ) call subroutine, returning capture groups
       (?&name(grouplist))   )
       (?P>name(grouplist))  )

       The list of capture groups "grouplist" is a comma-separated list of
       (absolute or relative) group numbers, and group names enclosed in
       single quotes or angle brackets.

       Here is an example which first uses the DEFINE condition to create a
       re-usable routine for matching a weekday, then calls that subroutine
       and retains the groups it captures for use later:

       (?x: # ignore whitespace for clarity
         # Define the routine "weekendday" which matches Saturday or
         # Sunday, and returns the Sat/Sun prefix as \k<short>.
         (?(DEFINE) (?<weekendday>
             (?|(?<short>Sat)urday|(?<short>Sun)day) ) )
         # Call the routine. Matches "Saturday,Sat" or "Sunday,Sun".
         (?&weekendday(<short>)),\k<short> )

       This feature is not available using the Oniguruma syntax \g<...> or
       \g'...' below.

   Oniguruma subroutine syntax
       For compatibility with Oniguruma, the non-Perl syntax \g followed by a
       name or a number enclosed either in angle brackets or single quotes, is
       an alternative syntax for calling a group as a subroutine, possibly
       recursively. Here are two of the examples used above, rewritten using
       this syntax:

       (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
       (sens|respons)e and \g'1'ibility

       PCRE2 supports an extension to Oniguruma: if a number is preceded by a
       plus or a minus sign it is taken as a relative reference. For example:

       (abc)(?i:\g<-1>)

       Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
       synonymous. The former is a backreference; the latter is a subroutine
       call.


CALLOUTS
       Perl has a feature whereby using the sequence (?{...}) causes arbitrary
       Perl code to be obeyed in the middle of matching a regular expression.
       This makes it possible, amongst other things, to extract different
       substrings that match the same pair of parentheses when there is a
       repetition.

       PCRE2 provides a similar feature, but of course it cannot obey
       arbitrary Perl code. The feature is called "callout". The caller of
       PCRE2 provides an external function by putting its entry point in a
       match context using the function pcre2_set_callout(), and then passing
       that context to pcre2_match() or pcre2_dfa_match(). If no match context
       is passed, or if the callout entry point is set to NULL, callout points
       will be passed over silently during matching. To disallow callouts in
       the pattern syntax, you may use the PCRE2_EXTRA_NEVER_CALLOUT option.

       Within a regular expression, (?C<arg>) indicates a point at which the
       external function is to be called. There are two kinds of callout:
       those with a numerical argument and those with a string argument. (?C)
       on its own with no argument is treated as (?C0). A numerical argument
       allows the application to distinguish between different callouts.
       String arguments were added for release 10.20 to make it possible for
       script languages that use PCRE2 to embed short scripts within patterns
       in a similar way to Perl.

       During matching, when PCRE2 reaches a callout point, the external
       function is called. It is provided with the number or string argument
       of the callout, the position in the pattern, and one item of data that
       is also set in the match block. The callout function may cause matching
       to proceed, to backtrack, or to fail.

       By default, PCRE2 implements a number of optimizations at matching
       time, and one side-effect is that sometimes callouts are skipped. If
       you need all possible callouts to happen, you need to set options that
       disable the relevant optimizations. More details, including a complete
       description of the programming interface to the callout function, are
       given in the pcre2callout documentation.

   Callouts with numerical arguments
       If you just want to have a means of identifying different callout
       points, put a number less than 256 after the letter C. For example,
       this pattern has two callout points:

       (?C1)abc(?C2)def

       If the PCRE2_AUTO_CALLOUT flag is passed to pcre2_compile(), numerical
       callouts are automatically installed before each item in the pattern.
       They are all numbered 255. If there is a conditional group in the
       pattern whose condition is an assertion, an additional callout is
       inserted just before the condition. An explicit callout may also be set
       at this position, as in this example:

       (?(?C9)(?=a)abc|def)

       Note that this applies only to assertion conditions, not to other types
       of condition.

   Callouts with string arguments
       A delimited string may be used instead of a number as a callout
       argument. The starting delimiter must be one of ` ' " ^ % # $ { and the
       ending delimiter is the same as the start, except for {, where the
       ending delimiter is }. If the ending delimiter is needed within the
       string, it must be doubled. For example:

       (?C'ab ''c'' d')xyz(?C{any text})pqr

       The doubling is removed before the string is passed to the callout
       function.


BACKTRACKING CONTROL
       There are a number of special "Backtracking Control Verbs" (to use
       Perl's terminology) that modify the behaviour of backtracking during
       matching. They are generally of the form (*VERB) or (*VERB:NAME). Some
       verbs take either form, and may behave differently depending on whether
       or not a name argument is present. The names are not required to be
       unique within the pattern.

       By default, for compatibility with Perl, a name is any sequence of
       characters that does not include a closing parenthesis. The name is not
       processed in any way, and it is not possible to include a closing
       parenthesis in the name. This can be changed by setting the
       PCRE2_ALT_VERBNAMES option, but the result is no longer
       Perl-compatible.

       When PCRE2_ALT_VERBNAMES is set, backslash processing is applied to
       verb names and only an unescaped closing parenthesis terminates the
       name. However, the only backslash items that are permitted are \Q, \E,
       and sequences such as \x{100} that define character code points.
       Character type escapes such as \d are faulted.

       A closing parenthesis can be included in a name either as \) or between
       \Q and \E. In addition to backslash processing, if the PCRE2_EXTENDED
       or PCRE2_EXTENDED_MORE option is also set, unescaped white space in
       verb names is skipped, and #-comments are recognized, exactly as in the
       rest of the pattern. PCRE2_EXTENDED and PCRE2_EXTENDED_MORE do not
       affect verb names unless PCRE2_ALT_VERBNAMES is also set.

       The maximum length of a name is 255 in the 8-bit library and 65535 in
       the 16-bit and 32-bit libraries. If the name is empty, that is, if the
       closing parenthesis immediately follows the colon, the effect is as if
       the colon were not there. Any number of these verbs may occur in a
       pattern. Except for (*ACCEPT), they may not be quantified.

       Since these verbs are specifically related to backtracking, most of
       them can be used only when the pattern is to be matched using the
       traditional matching function or JIT, because they use backtracking
       algorithms. With the exception of (*FAIL), which behaves like a failing
       negative assertion, the backtracking control verbs cause an error if
       encountered by the DFA matching function.

       The behaviour of these verbs in repeated groups, assertions, and in
       capture groups called as subroutines (whether or not recursively) is
       documented below.

   Optimizations that affect backtracking verbs
       PCRE2 contains some optimizations that are used to speed up matching by
       running some checks at the start of each match attempt. For example, it
       may know the minimum length of matching subject, or that a particular
       character must be present. When one of these optimizations bypasses the
       running of a match, any included backtracking verbs will not, of
       course, be processed. You can suppress the start-of-match optimizations
       by setting the PCRE2_NO_START_OPTIMIZE option when calling
       pcre2_compile(), by calling pcre2_set_optimize() with a
       PCRE2_START_OPTIMIZE_OFF directive, or by starting the pattern with
       (*NO_START_OPT). There is more discussion of this option in the section
       entitled "Compiling a pattern" in the pcre2api documentation.

       Experiments with Perl suggest that it too has similar optimizations,
       and like PCRE2, turning them off can change the result of a match.

   Verbs that act immediately
       The following verbs act as soon as they are encountered.

        (*ACCEPT) or (*ACCEPT:NAME)

       This verb causes the match to end successfully, skipping the remainder
       of the pattern. However, when it is inside a capture group that is
       called as a subroutine, only that group is ended successfully. Matching
       then continues at the outer level. If (*ACCEPT) in triggered in a
       positive assertion, the assertion succeeds; in a negative assertion,
       the assertion fails.

       If (*ACCEPT) is inside capturing parentheses, the data so far is
       captured. For example:

       A((?:A|B(*ACCEPT)|C)D)

       This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is
       captured by the outer parentheses.

       (*ACCEPT) is the only backtracking verb that is allowed to be
       quantified because an ungreedy quantification with a minimum of zero
       acts only when a backtrack happens. Consider, for example,

       (A(*ACCEPT)??B)C

       where A, B, and C may be complex expressions. After matching "A", the
       matcher processes "BC"; if that fails, causing a backtrack, (*ACCEPT)
       is triggered and the match succeeds. In both cases, all but C is
       captured. Whereas (*COMMIT) (see below) means "fail on backtrack", a
       repeated (*ACCEPT) of this type means "succeed on backtrack".

       Warning: (*ACCEPT) should not be used within a script run group,
       because it causes an immediate exit from the group, bypassing the
       script run checking.

       (*FAIL) or (*FAIL:NAME)

       This verb causes a matching failure, forcing backtracking to occur. It
       may be abbreviated to (*F). It is equivalent to (?!) but easier to
       read. The Perl documentation notes that it is probably useful only when
       combined with (?{}) or (??{}). Those are, of course, Perl features that
       are not present in PCRE2. The nearest equivalent is the callout
       feature, as for example in this pattern:

       a+(?C)(*FAIL)

       A match with the string "aaaa" always fails, but the callout is taken
       before each backtrack happens (in this example, 10 times).

       (*ACCEPT:NAME) and (*FAIL:NAME) behave the same as
       (*MARK:NAME)(*ACCEPT) and (*MARK:NAME)(*FAIL), respectively, that is, a
       (*MARK) is recorded just before the verb acts.

   Recording which path was taken
       There is one verb whose main purpose is to track how a match was
       arrived at, though it also has a secondary use in conjunction with
       advancing the match starting point (see (*SKIP) below).

       (*MARK:NAME) or (*:NAME)

       A name is always required with this verb. For all the other
       backtracking control verbs, a NAME argument is optional.

       When a match succeeds, the name of the last-encountered mark name on
       the matching path is passed back to the caller as described in the
       section entitled "Other information about the match" in the pcre2api
       documentation. This applies to all instances of (*MARK) and other
       verbs, including those inside assertions and atomic groups. However,
       there are differences in those cases when (*MARK) is used in
       conjunction with (*SKIP) as described below.

       The mark name that was last encountered on the matching path is passed
       back. A verb without a NAME argument is ignored for this purpose. Here
       is an example of pcre2test output, where the "mark" modifier requests
       the retrieval and outputting of (*MARK) data:

         re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
       data> XY
        0: XY
       MK: A
       XZ
        0: XZ
       MK: B

       The (*MARK) name is tagged with "MK:" in this output, and in this
       example it indicates which of the two alternatives matched. This is a
       more efficient way of obtaining this information than putting each
       alternative in its own capturing parentheses.

       If a verb with a name is encountered in a positive assertion that is
       true, the name is recorded and passed back if it is the
       last-encountered. This does not happen for negative assertions or
       failing positive assertions.

       After a partial match or a failed match, the last encountered name in
       the entire match process is returned. For example:

         re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
       data> XP
       No match, mark = B

       Note that in this unanchored example the mark is retained from the
       match attempt that started at the letter "X" in the subject. Subsequent
       match attempts starting at "P" and then with an empty string do not get
       as far as the (*MARK) item, but nevertheless do not reset it.

       If you are interested in (*MARK) values after failed matches, you
       should probably either set the PCRE2_NO_START_OPTIMIZE option or call
       pcre2_set_optimize() with a PCRE2_START_OPTIMIZE_OFF directive (see
       above) to ensure that the match is always attempted.

   Verbs that act after backtracking
       The following verbs do nothing when they are encountered. Matching
       continues with what follows, but if there is a subsequent match
       failure, causing a backtrack to the verb, a failure is forced. That is,
       backtracking cannot pass to the left of the verb. However, when one of
       these verbs appears inside an atomic group or in an atomic lookaround
       assertion that is true, its effect is confined to that group, because
       once the group has been matched, there is never any backtracking into
       it. Backtracking from beyond an atomic assertion or group ignores the
       entire group, and seeks a preceding backtracking point.

       These verbs differ in exactly what kind of failure occurs when
       backtracking reaches them. The behaviour described below is what
       happens when the verb is not in a subroutine or an assertion.
       Subsequent sections cover these special cases.

       (*COMMIT) or (*COMMIT:NAME)

       This verb causes the whole match to fail outright if there is a later
       matching failure that causes backtracking to reach it. Even if the
       pattern is unanchored, no further attempts to find a match by advancing
       the starting point take place. If (*COMMIT) is the only backtracking
       verb that is encountered, once it has been passed pcre2_match() is
       committed to finding a match at the current starting point, or not at
       all. For example:

       a+(*COMMIT)b

       This matches "xxaab" but not "aacaab". It can be thought of as a kind
       of dynamic anchor, or "I've started, so I must finish."

       The behaviour of (*COMMIT:NAME) is not the same as
       (*MARK:NAME)(*COMMIT). It is like (*MARK:NAME) in that the name is
       remembered for passing back to the caller. However, (*SKIP:NAME)
       searches only for names that are set with (*MARK), ignoring those set
       by any of the other backtracking verbs.

       If there is more than one backtracking verb in a pattern, a different
       one that follows (*COMMIT) may be triggered first, so merely passing
       (*COMMIT) during a match does not always guarantee that a match must be
       at this starting point.

       Note that (*COMMIT) at the start of a pattern is not the same as an
       anchor, unless PCRE2's start-of-match optimizations are turned off, as
       shown in this output from pcre2test:

         re> /(*COMMIT)abc/
       data> xyzabc
        0: abc
       data>
       re> /(*COMMIT)abc/no_start_optimize
       data> xyzabc
       No match

       For the first pattern, PCRE2 knows that any match must start with "a",
       so the optimization skips along the subject to "a" before applying the
       pattern to the first set of data. The match attempt then succeeds. The
       second pattern disables the optimization that skips along to the first
       character. The pattern is now applied starting at "x", and so the
       (*COMMIT) causes the match to fail without trying any other starting
       points.

       (*PRUNE) or (*PRUNE:NAME)

       This verb causes the match to fail at the current starting position in
       the subject if there is a later matching failure that causes
       backtracking to reach it. If the pattern is unanchored, the normal
       "bumpalong" advance to the next starting character then happens.
       Backtracking can occur as usual to the left of (*PRUNE), before it is
       reached, or when matching to the right of (*PRUNE), but if there is no
       match to the right, backtracking cannot cross (*PRUNE). In simple
       cases, the use of (*PRUNE) is just an alternative to an atomic group or
       possessive quantifier, but there are some uses of (*PRUNE) that cannot
       be expressed in any other way. In an anchored pattern (*PRUNE) has the
       same effect as (*COMMIT).

       The behaviour of (*PRUNE:NAME) is not the same as (*MARK:NAME)(*PRUNE).
       It is like (*MARK:NAME) in that the name is remembered for passing back
       to the caller. However, (*SKIP:NAME) searches only for names set with
       (*MARK), ignoring those set by other backtracking verbs.

       (*SKIP)

       This verb, when given without a name, is like (*PRUNE), except that if
       the pattern is unanchored, the "bumpalong" advance is not to the next
       character, but to the position in the subject where (*SKIP) was
       encountered. (*SKIP) signifies that whatever text was matched leading
       up to it cannot be part of a successful match if there is a later
       mismatch. Consider:

       a+(*SKIP)b

       If the subject is "aaaac...", after the first match attempt fails
       (starting at the first character in the string), the starting point
       skips on to start the next attempt at "c". Note that a possessive
       quantifier does not have the same effect as this example; although it
       would suppress backtracking during the first match attempt, the second
       attempt would start at the second character instead of skipping on to
       "c".

       If (*SKIP) is used to specify a new starting position that is the same
       as the starting position of the current match, or (by being inside a
       lookbehind) earlier, the position specified by (*SKIP) is ignored, and
       instead the normal "bumpalong" occurs.

       (*SKIP:NAME)

       When (*SKIP) has an associated name, its behaviour is modified. When
       such a (*SKIP) is triggered, the previous path through the pattern is
       searched for the most recent (*MARK) that has the same name. If one is
       found, the "bumpalong" advance is to the subject position that
       corresponds to that (*MARK) instead of to where (*SKIP) was
       encountered. If no (*MARK) with a matching name is found, the (*SKIP)
       is ignored.

       The search for a (*MARK) name uses the normal backtracking mechanism,
       which means that it does not see (*MARK) settings that are inside
       atomic groups or assertions, because they are never re-entered by
       backtracking. Compare the following pcre2test examples:

         re> /a(?>(*MARK:X))(*SKIP:X)(*F)|(.)/
       data: abc
        0: a
        1: a
       data:
         re> /a(?:(*MARK:X))(*SKIP:X)(*F)|(.)/
       data: abc
        0: b
        1: b

       In the first example, the (*MARK) setting is in an atomic group, so it
       is not seen when (*SKIP:X) triggers, causing the (*SKIP) to be ignored.
       This allows the second branch of the pattern to be tried at the first
       character position. In the second example, the (*MARK) setting is not
       in an atomic group. This allows (*SKIP:X) to find the (*MARK) when it
       backtracks, and this causes a new matching attempt to start at the
       second character. This time, the (*MARK) is never seen because "a" does
       not match "b", so the matcher immediately jumps to the second branch of
       the pattern.

       Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It
       ignores names that are set by other backtracking verbs.

       (*THEN) or (*THEN:NAME)

       This verb causes a skip to the next innermost alternative when
       backtracking reaches it. That is, it cancels any further backtracking
       within the current alternative. Its name comes from the observation
       that it can be used for a pattern-based if-then-else block:

       ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...

       If the COND1 pattern matches, FOO is tried (and possibly further items
       after the end of the group if FOO succeeds); on failure, the matcher
       skips to the second alternative and tries COND2, without backtracking
       into COND1. If that succeeds and BAR fails, COND3 is tried. If
       subsequently BAZ fails, there are no more alternatives, so there is a
       backtrack to whatever came before the entire group. If (*THEN) is not
       inside an alternation, it acts like (*PRUNE).

       The behaviour of (*THEN:NAME) is not the same as (*MARK:NAME)(*THEN).
       It is like (*MARK:NAME) in that the name is remembered for passing back
       to the caller. However, (*SKIP:NAME) searches only for names set with
       (*MARK), ignoring those set by other backtracking verbs.

       A group that does not contain a | character is just a part of the
       enclosing alternative; it is not a nested alternation with only one
       alternative. The effect of (*THEN) extends beyond such a group to the
       enclosing alternative. Consider this pattern, where A, B, etc. are
       complex pattern fragments that do not contain any | characters at this
       level:

       A (B(*THEN)C) | D

       If A and B are matched, but there is a failure in C, matching does not
       backtrack into A; instead it moves to the next alternative, that is, D.
       However, if the group containing (*THEN) is given an alternative, it
       behaves differently:

       A (B(*THEN)C | (*FAIL)) | D

       The effect of (*THEN) is now confined to the inner group. After a
       failure in C, matching moves to (*FAIL), which causes the whole group
       to fail because there are no more alternatives to try. In this case,
       matching does backtrack into A.

       Note that a conditional group is not considered as having two
       alternatives, because only one is ever used. In other words, the |
       character in a conditional group has a different meaning. Ignoring
       white space, consider:

       ^.*? (?(?=a) a | b(*THEN)c )

       If the subject is "ba", this pattern does not match. Because .? is
       ungreedy, it initially matches zero characters. The condition (?=a)
       then fails, the character "b" is matched, but "c" is not. At this
       point, matching does not backtrack to .? as might perhaps be expected
       from the presence of the | character. The conditional group is part of
       the single alternative that comprises the whole pattern, and so the
       match fails. (If there was a backtrack into .*?, allowing it to match
       "b", the match would succeed.)

       The verbs just described provide four different "strengths" of control
       when subsequent matching fails. (*THEN) is the weakest, carrying on the
       match at the next alternative. (*PRUNE) comes next, failing the match
       at the current starting position, but allowing an advance to the next
       character (for an unanchored pattern). (*SKIP) is similar, except that
       the advance may be more than one character. (*COMMIT) is the strongest,
       causing the entire match to fail.

   More than one backtracking verb
       If more than one backtracking verb is present in a pattern, the one
       that is backtracked onto first acts. For example, consider this
       pattern, where A, B, etc. are complex pattern fragments:

       (A(*COMMIT)B(*THEN)C|ABD)

       If A matches but B fails, the backtrack to (*COMMIT) causes the entire
       match to fail. However, if A and B match, but C fails, the backtrack to
       (*THEN) causes the next alternative (ABD) to be tried. This behaviour
       is consistent, but is not always the same as Perl's. It means that if
       two or more backtracking verbs appear in succession, all but the last
       of them has no effect. Consider this example:

       ...(*COMMIT)(*PRUNE)...

       If there is a matching failure to the right, backtracking onto (*PRUNE)
       causes it to be triggered, and its action is taken. There can never be
       a backtrack onto (*COMMIT).

   Backtracking verbs in repeated groups
       PCRE2 sometimes differs from Perl in its handling of backtracking verbs
       in repeated groups. For example, consider:

       /(a(*COMMIT)b)+ac/

       If the subject is "abac", Perl matches unless its optimizations are
       disabled, but PCRE2 always fails because the (*COMMIT) in the second
       repeat of the group acts.

   Backtracking verbs in assertions
       (*FAIL) in any assertion has its normal effect: it forces an immediate
       backtrack. The behaviour of the other backtracking verbs depends on
       whether or not the assertion is standalone or acting as the condition
       in a conditional group.

       (*ACCEPT) in a standalone positive assertion causes the assertion to
       succeed without any further processing; captured strings and a mark
       name (if set) are retained. In a standalone negative assertion,
       (*ACCEPT) causes the assertion to fail without any further processing;
       captured substrings and any mark name are discarded.

       If the assertion is a condition, (*ACCEPT) causes the condition to be
       true for a positive assertion and false for a negative one; captured
       substrings are retained in both cases.

       The remaining verbs act only when a later failure causes a backtrack to
       reach them. This means that, for the Perl-compatible assertions, their
       effect is confined to the assertion, because Perl lookaround assertions
       are atomic. A backtrack that occurs after such an assertion is complete
       does not jump back into the assertion. Note in particular that a
       (*MARK) name that is set in an assertion is not "seen" by an instance
       of (*SKIP:NAME) later in the pattern.

       PCRE2 now supports non-atomic positive assertions and also "scan
       substring" assertions, as described in the sections entitled
       "Non-atomic assertions" and "Scan substring assertions" above. These
       assertions must be standalone (not used as conditions). They are not
       Perl-compatible. For these assertions, a later backtrack does jump back
       into the assertion, and therefore verbs such as (*COMMIT) can be
       triggered by backtracks from later in the pattern.

       The effect of (*THEN) is not allowed to escape beyond an assertion. If
       there are no more branches to try, (*THEN) causes a positive assertion
       to be false, and a negative assertion to be true. This behaviour
       differs from Perl when the assertion has only one branch.

       The other backtracking verbs are not treated specially if they appear
       in a standalone positive assertion. In a conditional positive
       assertion, backtracking (from within the assertion) into (*COMMIT),
       (*SKIP), or (*PRUNE) causes the condition to be false. However, for
       both standalone and conditional negative assertions, backtracking into
       (*COMMIT), (*SKIP), or (*PRUNE) causes the assertion to be true,
       without considering any further alternative branches.

   Backtracking verbs in subroutines
       These behaviours occur whether or not the group is called recursively.

       (*ACCEPT) in a group called as a subroutine causes the subroutine match
       to succeed without any further processing. Matching then continues
       after the subroutine call. Perl documents this behaviour. Perl's
       treatment of the other verbs in subroutines is different in some cases.

       (*FAIL) in a group called as a subroutine has its normal effect: it
       forces an immediate backtrack.

       (*COMMIT), (*SKIP), and (*PRUNE) cause the subroutine match to fail
       when triggered by being backtracked to in a group called as a
       subroutine. There is then a backtrack at the outer level.

       (*THEN), when triggered, skips to the next alternative in the innermost
       enclosing group that has alternatives (its normal behaviour). However,
       if there is no such group within the subroutine's group, the subroutine
       match fails and there is a backtrack at the outer level.


EBCDIC ENVIRONMENTS
       Differences in the way PCRE behaves when it is running in an EBCDIC
       environment are covered in this section.

   Escape sequences
       When PCRE2 is compiled in EBCDIC mode, \N{U+hhh..} is not supported.
       \a, \e, \f, \n, \r, and \t generate the appropriate EBCDIC code values.
       The \c escape is processed as specified for Perl in the perlebcdic
       document. The only characters that are allowed after \c are A-Z, a-z,
       or one of @, [, \, ], ^, _, or ?. Any other character provokes a
       compile-time error. The sequence \c@ encodes character code 0; after \c
       the letters (in either case) encode characters 1-26 (hex 01 to hex 1A);
       [, \, ], ^, and _ encode characters 27-31 (hex 1B to hex 1F), and \c?
       becomes either 255 (hex FF) or 95 (hex 5F).

       Thus, apart from \c?, these escapes generate the same character code
       values as they do in an ASCII or Unicode environment, though the
       meanings of the values mostly differ. For example, \cG always generates
       code value 7, which is BEL in ASCII but DEL in EBCDIC.

       The sequence \c? generates DEL (127, hex 7F) in an ASCII environment,
       but because 127 is not a control character in EBCDIC, Perl makes it
       generate the APC character. Unfortunately, there are several variants
       of EBCDIC. In most of them the APC character has the value 255 (hex
       FF), but in the one Perl calls POSIX-BC its value is 95 (hex 5F). If
       certain other characters have POSIX-BC values, PCRE2 makes \c? generate
       95; otherwise it generates 255.

   Character classes
       In character classes there is a special case in EBCDIC environments for
       ranges whose end points are both specified as literal letters in the
       same case. For compatibility with Perl, EBCDIC code points within the
       range that are not letters are omitted. For example, [h-k] matches only
       four characters, even though the EBCDIC codes for h and k are 0x88 and
       0x92, a range of 11 code points. However, if the range is specified
       numerically, for example, [\x88-\x92] or [h-\x92], all code points are
       included.


SEE ALSO
       pcre2api(3), pcre2callout(3), pcre2matching(3), pcre2syntax(3),
       pcre2(3).


AUTHOR
         Philip Hazel
         Retired from University Computing Service
         Cambridge, England.


REVISION
       Last updated: 25 October 2025
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.49-DEV                 25 October 2025                PCRE2PATTERN(3)
------------------------------------------------------------------------------


PCRE2PERFORM(3)                      PCRE2                     PCRE2PERFORM(3)


NAME
       pcre2perform - PCRE2 performance


PCRE2 PERFORMANCE
       Two aspects of performance are discussed below: memory usage and
       processing time. The way you express your pattern as a regular
       expression can affect both of them.


COMPILED PATTERN MEMORY USAGE
       Patterns are compiled by PCRE2 into a reasonably efficient interpretive
       code, so that most simple patterns do not use much memory for storing
       the compiled version. However, there is one case where the memory usage
       of a compiled pattern can be unexpectedly large. If a parenthesized
       group has a quantifier with a minimum greater than 1 and/or a limited
       maximum, the whole group is repeated in the compiled code. For example,
       the pattern

       (abc|def){2,4}

       is compiled as if it were

       (abc|def)(abc|def)((abc|def)(abc|def)?)?

       (Technical aside: It is done this way so that backtrack points within
       each of the repetitions can be independently maintained.)

       For regular expressions whose quantifiers use only small numbers, this
       is not usually a problem. However, if the numbers are large, and
       particularly if such repetitions are nested, the memory usage can
       become an embarrassment. For example, the very simple pattern

       ((ab){1,1000}c){1,3}

       uses over 50KiB when compiled using the 8-bit library. When PCRE2 is
       compiled with its default internal pointer size of two bytes, the size
       limit on a compiled pattern is 65535 code units in the 8-bit and 16-bit
       libraries, and this is reached with the above pattern if the outer
       repetition is increased from 3 to 4. PCRE2 can be compiled to use
       larger internal pointers and thus handle larger compiled patterns, but
       it is better to try to rewrite your pattern to use less memory if you
       can.

       One way of reducing the memory usage for such patterns is to make use
       of PCRE2's "subroutine" facility. Re-writing the above pattern as

       ((ab)(?2){0,999}c)(?1){0,2}

       reduces the memory requirements to around 16KiB, and indeed it remains
       under 20KiB even with the outer repetition increased to 100. However,
       this kind of pattern is not always exactly equivalent, because any
       captures within subroutine calls are lost when the subroutine
       completes. If this is not a problem, this kind of rewriting will allow
       you to process patterns that PCRE2 cannot otherwise handle. The
       matching performance of the two different versions of the pattern are
       roughly the same. (This applies from release 10.30 - things were
       different in earlier releases.)


STACK AND HEAP USAGE AT RUN TIME
       From release 10.30, the interpretive (non-JIT) version of pcre2_match()
       uses very little system stack at run time. In earlier releases
       recursive function calls could use a great deal of stack, and this
       could cause problems, but this usage has been eliminated. Backtracking
       positions are now explicitly remembered in memory frames controlled by
       the code.

       The size of each frame depends on the size of pointer variables and the
       number of capturing parenthesized groups in the pattern being matched.
       On a 64-bit system the frame size for a pattern with no captures is 128
       bytes. For each capturing group the size increases by 16 bytes.

       Until release 10.41, an initial 20KiB frames vector was allocated on
       the system stack, but this still caused some issues for multi-thread
       applications where each thread has a very small stack. From release
       10.41 backtracking memory frames are always held in heap memory. An
       initial heap allocation is obtained the first time any match data block
       is passed to pcre2_match(). This is remembered with the match data
       block and re-used if that block is used for another match. It is freed
       when the match data block itself is freed.

       The size of the initial block is the larger of 20KiB or ten times the
       pattern's frame size, unless the heap limit is less than this, in which
       case the heap limit is used. If the initial block proves to be too
       small during matching, it is replaced by a larger block, subject to the
       heap limit. The heap limit is checked only when a new block is to be
       allocated. Reducing the heap limit between calls to pcre2_match() with
       the same match data block does not affect the saved block.

       In contrast to pcre2_match(), pcre2_dfa_match() does use recursive
       function calls, but only for processing atomic groups, lookaround
       assertions, and recursion within the pattern. The original version of
       the code used to allocate quite large internal workspace vectors on the
       stack, which caused some problems for some patterns in environments
       with small stacks. From release 10.32 the code for pcre2_dfa_match()
       has been re-factored to use heap memory when necessary for internal
       workspace when recursing, though recursive function calls are still
       used.

       The "match depth" parameter can be used to limit the depth of function
       recursion, and the "match heap" parameter to limit heap memory in
       pcre2_dfa_match().


PROCESSING TIME
       Certain items in regular expression patterns are processed more
       efficiently than others. It is more efficient to use a character class
       like [aeiou] than a set of single-character alternatives such as
       (a|e|i|o|u). In general, the simplest construction that provides the
       required behaviour is usually the most efficient. Jeffrey Friedl's book
       contains a lot of useful general discussion about optimizing regular
       expressions for efficient performance. This document contains a few
       observations about PCRE2.

       Using Unicode character properties (the \p, \P, and \X escapes) is
       slow, because PCRE2 has to use a multi-stage table lookup whenever it
       needs a character's property. If you can find an alternative pattern
       that does not use character properties, it will probably be faster.

       By default, the escape sequences \b, \d, \s, and \w, and the POSIX
       character classes such as [:alpha:] do not use Unicode properties,
       partly for backwards compatibility, and partly for performance reasons.
       However, you can set the PCRE2_UCP option or start the pattern with
       (*UCP) if you want Unicode character properties to be used. This can
       double the matching time for items such as \d, when matched with
       pcre2_match(); the performance loss is less with a DFA matching
       function, and in both cases there is not much difference for \b.

       When a pattern begins with .* not in atomic parentheses, nor in
       parentheses that are the subject of a backreference, and the
       PCRE2_DOTALL option is set, the pattern is implicitly anchored by
       PCRE2, since it can match only at the start of a subject string. If the
       pattern has multiple top-level branches, they must all be anchorable.
       The optimization can be disabled by the PCRE2_NO_DOTSTAR_ANCHOR option,
       and is automatically disabled if the pattern contains (*PRUNE) or
       (*SKIP).

       If PCRE2_DOTALL is not set, PCRE2 cannot make this optimization,
       because the dot metacharacter does not then match a newline, and if the
       subject string contains newlines, the pattern may match from the
       character immediately following one of them instead of from the very
       start. For example, the pattern

       .*second

       matches the subject "first\nand second" (where \n stands for a newline
       character), with the match starting at the seventh character. In order
       to do this, PCRE2 has to retry the match starting after every newline
       in the subject.

       If you are using such a pattern with subject strings that do not
       contain newlines, the best performance is obtained by setting
       PCRE2_DOTALL, or starting the pattern with ^. or ^.? to indicate
       explicit anchoring. That saves PCRE2 from having to scan along the
       subject looking for a newline to restart at.

       Beware of patterns that contain nested indefinite repeats. These can
       take a long time to run when applied to a string that does not match.
       Consider the pattern fragment

       ^(a+)*

       This can match "aaaa" in 16 different ways, and this number increases
       very rapidly as the string gets longer. (The * repeat can match 0, 1,
       2, 3, or 4 times, and for each of those cases other than 0 or 4, the +
       repeats can match different numbers of times.) When the remainder of
       the pattern is such that the entire match is going to fail, PCRE2 has
       in principle to try every possible variation, and this can take an
       extremely long time, even for relatively short strings.

       An optimization catches some of the more simple cases such as

       (a+)*b

       where a literal character follows. Before embarking on the standard
       matching procedure, PCRE2 checks that there is a "b" later in the
       subject string, and if there is not, it fails the match immediately.
       However, when there is no following literal this optimization cannot be
       used. You can see the difference by comparing the behaviour of

       (a+)*\d

       with the pattern above. The former gives a failure almost instantly
       when applied to a whole line of "a" characters, whereas the latter
       takes an appreciable time with strings longer than about 20 characters.

       In many cases, the solution to this kind of performance issue is to use
       an atomic group or a possessive quantifier. This can often reduce
       memory requirements as well. As another example, consider this pattern:

       ([^<]|<(?!inet))+

       It matches from wherever it starts until it encounters "<inet" or the
       end of the data, and is the kind of pattern that might be used when
       processing an XML file. Each iteration of the outer parentheses matches
       either one character that is not "<" or a "<" that is not followed by
       "inet". However, each time a parenthesis is processed, a backtracking
       position is passed, so this formulation uses a memory frame for each
       matched character. For a long string, a lot of memory is required.
       Consider now this rewritten pattern, which matches exactly the same
       strings:

       ([^<]++|<(?!inet))+

       This runs much faster, because sequences of characters that do not
       contain "<" are "swallowed" in one item inside the parentheses, and a
       possessive quantifier is used to stop any backtracking into the runs of
       non-"<" characters. This version also uses a lot less memory because
       entry to a new set of parentheses happens only when a "<" character
       that is not followed by "inet" is encountered (and we assume this is
       relatively rare).

       This example shows that one way of optimizing performance when matching
       long subject strings is to write repeated parenthesized subpatterns to
       match more than one character whenever possible.

   Setting resource limits
       You can set limits on the amount of processing that takes place when
       matching, and on the amount of heap memory that is used. The default
       values of the limits are very large, and unlikely ever to operate. They
       can be changed when PCRE2 is built, and they can also be set when
       pcre2_match() or pcre2_dfa_match() is called. For details of these
       interfaces, see the pcre2build documentation and the section entitled
       "The match context" in the pcre2api documentation.

       The pcre2test test program has a modifier called "find_limits" which,
       if applied to a subject line, causes it to find the smallest limits
       that allow a pattern to match. This is done by repeatedly matching with
       different limits.


AUTHOR
         Philip Hazel
         Retired from University Computing Service
         Cambridge, England.


REVISION
       Last updated: 06 December 2022
       Copyright (c) 1997-2022 University of Cambridge.


PCRE2 10.49-DEV                06 December 2022                PCRE2PERFORM(3)
------------------------------------------------------------------------------


PCRE2POSIX(3)                        PCRE2                       PCRE2POSIX(3)


NAME
       pcre2posix - PCRE2 POSIX API


SYNOPSIS
       #include <pcre2posix.h>

       int pcre2_regcomp(regex_t *preg, const char *pattern,
            int cflags);

       int pcre2_regexec(const regex_t *preg, const char *string,
            size_t nmatch, regmatch_t pmatch[], int eflags);

       size_t pcre2_regerror(int errcode, const regex_t *preg,
            char *errbuf, size_t errbuf_size);

       void pcre2_regfree(regex_t *preg);


DESCRIPTION
       This set of functions provides a POSIX-style API for the PCRE2 regular
       expression 8-bit library. There are no POSIX-style wrappers for PCRE2's
       16-bit and 32-bit libraries. See the pcre2api documentation for a
       description of PCRE2's native API, which contains much additional
       functionality.

           Important

           The functions described here are NOT thread-safe, and should not be
           used in multi-threaded applications. They are also limited to
           processing subjects that are not bigger than 2GB. Use the native
           API instead.

       These functions are wrapper functions that ultimately call the PCRE2
       native API. Their prototypes are defined in the pcre2posix.h header
       file, and they all have unique names starting with pcre2_. However, the
       pcre2posix.h header also contains macro definitions that convert the
       standard POSIX names such as regcomp() into pcre2_regcomp() etc. This
       means that a program can use the usual POSIX names without running the
       risk of accidentally linking with POSIX functions from a different
       library.

       On Unix-like systems the PCRE2 POSIX library is called libpcre2-posix,
       so can be accessed by adding -lpcre2-posix to the command for linking
       an application. Because the POSIX functions call the native ones, it is
       also necessary to add -lpcre2-8.

       On Windows systems, if you are linking to a DLL version of the library,
       it is recommended that PCRE2POSIX_SHARED is defined before including
       the pcre2posix.h header, as it will allow for a more efficient way to
       invoke the functions by adding the __declspec(dllimport) decorator.

       Although they were not defined as prototypes in pcre2posix.h, releases
       10.33 to 10.36 of the library contained functions with the POSIX names
       regcomp() etc. These simply passed their arguments to the PCRE2
       functions. These functions were provided for backwards compatibility
       with earlier versions of PCRE2, which had only POSIX names. However,
       this has proved troublesome in situations where a program links with
       several libraries, some of which use PCRE2's POSIX interface while
       others use the real POSIX functions. For this reason, the POSIX names
       have been removed since release 10.37.

       Calling the header file pcre2posix.h avoids any conflict with other
       POSIX libraries. It can, of course, be renamed or aliased as regex.h,
       which is the "correct" name, if there is no clash. It provides two
       structure types, type:regex_t for compiled internal forms, and
       type:regmatch_t for returning captured substrings. It also defines some
       constants whose names start with "REG_"; these are used for setting
       options and identifying error codes.


USING THE POSIX FUNCTIONS
       Note that these functions are just POSIX-style wrappers for PCRE2's
       native API. They do not give POSIX regular expression behaviour, and
       they are not thread-safe or even POSIX compatible.

       Those POSIX option bits that can reasonably be mapped to PCRE2 native
       options have been implemented. In addition, the option
       const:REG_EXTENDED is defined with the value zero. This has no effect,
       but since programs that are written to the POSIX interface often use
       it, this makes it easier to slot in PCRE2 as a replacement library.
       Other POSIX options are not even defined.

       There are also some options that are not defined by POSIX. These have
       been added at the request of users who want to make use of certain
       PCRE2-specific features via the POSIX calling interface or to add BSD
       or GNU functionality.

       When PCRE2 is called via these functions, it is only the API that is
       POSIX-like in style. The syntax and semantics of the regular
       expressions themselves are still those of Perl, subject to the setting
       of various PCRE2 options, as described below. "POSIX-like in style"
       means that the API approximates to the POSIX definition; it is not
       fully POSIX-compatible, and in multi-unit encoding domains it is
       probably even less compatible.

       The descriptions below use the actual names of the functions, but, as
       described above, the standard POSIX names (without the pcre2_ prefix)
       may also be used.


COMPILING A PATTERN
       The function pcre2_regcomp() is called to compile a pattern into an
       internal form. By default, the pattern is a C string terminated by a
       binary zero (but see const:REG_PEND below). The preg argument is a
       pointer to a type:regex_t structure that is used as a base for storing
       information about the compiled regular expression. It is also used for
       input when REG_PEND is set. The type:regex_t structure used by
       pcre2_regcomp() is defined in pcre2posix.h and is not the same as the
       structure used by other libraries that provide POSIX-style matching.

       The argument cflags is either zero, or contains one or more of the bits
       defined by the following macros:

           const:REG_DOTALL   The PCRE2_DOTALL option is set when the regular
                              expression is passed for compilation to the
                              native function. Note that const:REG_DOTALL is
                              not part of the POSIX standard.
           const:REG_ICASE    The PCRE2_CASELESS option is set when the
                              regular expression is passed for compilation to
                              the native function.
           const:REG_NEWLINE  The PCRE2_MULTILINE option is set when the
                              regular expression is passed for compilation to
                              the native function. Note that this does not
                              mimic the defined POSIX behaviour for
                              const:REG_NEWLINE (see the following section).
           const:REG_NOSPEC   The PCRE2_LITERAL option is set when the regular
                              expression is passed for compilation to the
                              native function. This disables all meta
                              characters in the pattern, causing it to be
                              treated as a literal string. The only other
                              options that are allowed with const:REG_NOSPEC
                              are const:REG_ICASE, const:REG_NOSUB,
                              const:REG_PEND, and const:REG_UTF. Note that
                              const:REG_NOSPEC is not part of the POSIX
                              standard.
           const:REG_NOSUB    When a pattern that is compiled with this flag
                              is passed to pcre2_regexec() for matching, the
                              nmatch and pmatch arguments are ignored, and no
                              captured strings are returned. Versions of the
                              PCRE2 library prior to 10.22 used to set the
                              PCRE2_NO_AUTO_CAPTURE compile option, but this
                              no longer happens because it disables the use of
                              backreferences.
           const:REG_PEND     If this option is set, the reg_endp field in the
                              preg structure (which has the type const char *)
                              must be set to point to the character beyond the
                              end of the pattern before calling
                              pcre2_regcomp(). The pattern itself may now
                              contain binary zeros, which are treated as data
                              characters. Without const:REG_PEND, a binary
                              zero terminates the pattern and the re_endp
                              field is ignored. This is a GNU extension to the
                              POSIX standard and should be used with caution
                              in software intended to be portable to other
                              systems.
           const:REG_UCP      The PCRE2_UCP option is set when the regular
                              expression is passed for compilation to the
                              native function. This causes PCRE2 to use
                              Unicode properties when matching \d, \w, etc.,
                              instead of just recognizing ASCII values. Note
                              that const:REG_UCP is not part of the POSIX
                              standard.
           const:REG_UNGREEDY The PCRE2_UNGREEDY option is set when the
                              regular expression is passed for compilation to
                              the native function. Note that
                              const:REG_UNGREEDY is not part of the POSIX
                              standard.
           const:REG_UTF      The PCRE2_UTF option is set when the regular
                              expression is passed for compilation to the
                              native function. This causes the pattern itself
                              and all data strings used for matching it to be
                              treated as UTF-8 strings. Note that
                              const:REG_UTF is not part of the POSIX standard.

       In the absence of these flags, no options are passed to the native
       function. This means that the regex is compiled with PCRE2 default
       semantics. In particular, the way it handles newline characters in the
       subject string is the Perl way, not the POSIX way. Note that setting
       PCRE2_MULTILINE has only some of the effects specified for
       const:REG_NEWLINE. It does not affect the way newlines are matched by
       the dot metacharacter (they are not) or by a negative class such as
       [^a] (they are).

       The yield of pcre2_regcomp() is zero on success, and non-zero
       otherwise. The preg structure is filled in on success, and one other
       member of the structure (as well as re_endp) is public: re_nsub
       contains the number of capturing subpatterns in the regular expression.
       Various error codes are defined in the header file.

           Note

           If the yield of pcre2_regcomp() is non-zero, you must not attempt
           to use the contents of the preg structure. If, for example, you
           pass it to pcre2_regexec(), the result is undefined and your
           program is likely to crash.


MATCHING NEWLINE CHARACTERS
       This area is not simple, because POSIX and Perl take different views of
       things. It is not possible to get PCRE2 to obey POSIX semantics, but
       then PCRE2 was never intended to be a POSIX engine. The following table
       lists the different possibilities for matching newline characters in
       Perl and PCRE2:

                                 Default   Change with

         . matches newline          no     PCRE2_DOTALL
         newline matches [^a]       yes    not changeable
         $ matches \n at end        yes    PCRE2_DOLLAR_ENDONLY
         $ matches \n in middle     no     PCRE2_MULTILINE
         ^ matches \n in middle     no     PCRE2_MULTILINE

       This is the equivalent table for a POSIX-compatible pattern matcher:

                                 Default   Change with

         . matches newline          yes    REG_NEWLINE
         newline matches [^a]       yes    REG_NEWLINE
         $ matches \n at end        no     REG_NEWLINE
         $ matches \n in middle     no     REG_NEWLINE
         ^ matches \n in middle     no     REG_NEWLINE

       This behaviour is not what happens when PCRE2 is called via its POSIX
       API. By default, PCRE2's behaviour is the same as Perl's, except that
       there is no equivalent for PCRE2_DOLLAR_ENDONLY in Perl. In both PCRE2
       and Perl, there is no way to stop newline from matching [^a].

       Default POSIX newline handling can be obtained by setting PCRE2_DOTALL
       and PCRE2_DOLLAR_ENDONLY when calling pcre2_compile() directly, but
       there is no way to make PCRE2 behave exactly as for the
       const:REG_NEWLINE action. When using the POSIX API, passing
       const:REG_NEWLINE to PCRE2's pcre2_regcomp() function causes
       PCRE2_MULTILINE to be passed to pcre2_compile(), and const:REG_DOTALL
       passes PCRE2_DOTALL. There is no way to pass PCRE2_DOLLAR_ENDONLY.


MATCHING A PATTERN
       The function pcre2_regexec() is called to match a compiled pattern preg
       against a given string, which is by default terminated by a zero byte
       (but see const:REG_STARTEND below), subject to the options in eflags.
       These can be:

           const:REG_NOTBOL   The PCRE2_NOTBOL option is set when calling the
                              underlying PCRE2 matching function.
           const:REG_NOTEMPTY The PCRE2_NOTEMPTY option is set when calling
                              the underlying PCRE2 matching function. Note
                              that const:REG_NOTEMPTY is not part of the POSIX
                              standard. However, setting this option can give
                              more POSIX-like behaviour in some situations.
           const:REG_NOTEOL   The PCRE2_NOTEOL option is set when calling the
                              underlying PCRE2 matching function.
           const:REG_STARTEND
                                   When this option is set, the subject string
                              starts at string + pmatch[0].rm_so and ends at
                              string + pmatch[0].rm_eo, which should point to
                              the first character beyond the string. There may
                              be binary zeros within the subject string, and
                              indeed, using const:REG_STARTEND is the only way
                              to pass a subject string that contains a binary
                              zero.

       Whatever the value of pmatch[0].rm_so, the offsets of the matched
       string and any captured substrings are still given relative to the
       start of string itself. (Before PCRE2 release 10.30 these were given
       relative to string + pmatch[0].rm_so, but this differs from other
       implementations.)

       This is a BSD extension, compatible with but not specified by IEEE
       Standard 1003.2 (POSIX.2), and should be used with caution in software
       intended to be portable to other systems. Note that a non-zero rm_so
       does not imply const:REG_NOTBOL; const:REG_STARTEND affects only the
       location and length of the string, not how it is matched. Setting
       const:REG_STARTEND and passing pmatch as NULL are mutually exclusive;
       the error const:REG_INVARG is returned.

       If the pattern was compiled with the const:REG_NOSUB flag, no data
       about any matched strings is returned. The nmatch and pmatch arguments
       of pcre2_regexec() are ignored (except possibly as input for
       const:REG_STARTEND).

       The value of nmatch may be zero, and the value pmatch may be NULL
       (unless const:REG_STARTEND is set); in both these cases no data about
       any matched strings is returned.

       Otherwise, the portion of the string that was matched, and also any
       captured substrings, are returned via the pmatch argument, which points
       to an array of nmatch structures of type type:regmatch_t, containing
       the members rm_so and rm_eo. These contain the byte offset to the first
       character of each substring and the offset to the first character after
       the end of each substring, respectively. The 0th element of the vector
       relates to the entire portion of string that was matched; subsequent
       elements relate to the capturing subpatterns of the regular expression.
       Unused entries in the array have both structure members set to -1.

       type:regmatch_t as well as the type:regoff_t typedef it uses are
       defined in pcre2posix.h and are not warranted to have the same size or
       layout as other similarly named types from other libraries that provide
       POSIX-style matching.

       A successful match yields a zero return; various error codes are
       defined in the header file, of which const:REG_NOMATCH is the
       "expected" failure code.


ERROR MESSAGES
       The pcre2_regerror() function maps a non-zero errorcode from either
       pcre2_regcomp() or pcre2_regexec() to a printable message. If preg is
       not NULL, the error should have arisen from the use of that structure.
       A message terminated by a binary zero is placed in errbuf. If the
       buffer is too short, only the first errbuf_size - 1 characters of the
       error message are used. The yield of the function is the size of buffer
       needed to hold the whole message, including the terminating zero. This
       value is greater than errbuf_size if the message was truncated.


MEMORY USAGE
       Compiling a regular expression causes memory to be allocated and
       associated with the preg structure. The function pcre2_regfree() frees
       all such memory, after which preg may no longer be used as a compiled
       expression.


AUTHOR
         Philip Hazel
         Retired from University Computing Service
         Cambridge, England.


REVISION
       Last updated: 27 November 2024
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.49-DEV                27 November 2024                  PCRE2POSIX(3)
------------------------------------------------------------------------------


PCRE2SAMPLE(3)                       PCRE2                      PCRE2SAMPLE(3)


NAME
       pcre2sample - PCRE2 sample program


PCRE2 SAMPLE PROGRAM
       A simple, complete demonstration program to get you started with using
       PCRE2 is supplied in the file pcre2demo.c in the src directory in the
       PCRE2 distribution. A listing of this program is given in the pcre2demo
       documentation. If you do not have a copy of the PCRE2 distribution, you
       can save this listing to recreate the contents of pcre2demo.c.

       The demonstration program compiles the regular expression that is its
       first argument, and matches it against the subject string in its second
       argument. No PCRE2 options are set, and default character tables are
       used. If matching succeeds, the program outputs the portion of the
       subject that matched, together with the contents of any captured
       substrings.

       If the -g option is given on the command line, the program then goes on
       to check for further matches of the same regular expression in the same
       subject string. The logic is a little bit tricky because of the
       possibility of matching an empty string. Comments in the code explain
       what is going on.

       The code in pcre2demo.c is an 8-bit program that uses the PCRE2 8-bit
       library. It handles strings and characters that are stored in 8-bit
       code units. By default, one character corresponds to one code unit, but
       if the pattern starts with (*UTF), both it and the subject are treated
       as UTF-8 strings, where characters may occupy multiple code units.

       If PCRE2 is installed in the standard include and library directories
       for your operating system, you should be able to compile the
       demonstration program using a command like this:

       cc -o pcre2demo pcre2demo.c -lpcre2-8

       If PCRE2 is installed elsewhere, you may need to add additional options
       to the command line. For example, on a Unix-like system that has PCRE2
       installed in /usr/local, you can compile the demonstration program
       using a command like this:

       cc -o pcre2demo -I/usr/local/include pcre2demo.c \
          -L/usr/local/lib -lpcre2-8

       Once you have built the demonstration program, you can run simple tests
       like this:

       ./pcre2demo 'cat|dog' 'the cat sat on the mat'
       ./pcre2demo -g 'cat|dog' 'the dog sat on the cat'
       ./pcre2demo -i 'cat' 'the dog sat on the CAT'

       Note that there is a much more comprehensive test program, called
       pcre2test, which supports many more facilities for testing regular
       expressions using all three PCRE2 libraries (8-bit, 16-bit, and 32-bit,
       though not all three need be installed). The pcre2demo program is
       provided as a relatively simple coding example.

       If you try to run pcre2demo when PCRE2 is not installed in the standard
       library directory, you may get an error like this on some operating
       systems (e.g. Solaris):

         ld.so.1: pcre2demo: fatal: libpcre2-8.so.0: open failed: No such file
       or directory

       This is caused by the way shared library support works on those
       systems. You need to add

       -R/usr/local/lib

       (for example) to the compile command to get round this problem.


AUTHOR
         Philip Hazel
         Retired from University Computing Service
         Cambridge, England.


REVISION
       Last updated: 28 February 2025
       Copyright (c) 1997-2016 University of Cambridge.


PCRE2 10.49-DEV                28 February 2025                 PCRE2SAMPLE(3)
------------------------------------------------------------------------------
PCRE2SERIALIZE(3)                    PCRE2                   PCRE2SERIALIZE(3)


NAME
       pcre2serialize - PCRE2 serialized patterns


SAVING AND RE-USING PRECOMPILED PCRE2 PATTERNS
       int32_t pcre2_serialize_decode(pcre2_code **codes,
                                      int32_t number_of_codes,
                                      const uint8_t *bytes,
                                      pcre2_general_context *gcontext);

       int32_t pcre2_serialize_encode(const pcre2_code **codes,
                                      int32_t number_of_codes,
                                      uint8_t **serialized_bytes,
                                      PCRE2_SIZE *serialized_size,
                                      pcre2_general_context *gcontext);

       void pcre2_serialize_free(uint8_t *bytes);

       int32_t pcre2_serialize_get_number_of_codes(const uint8_t *bytes);

       If you are running an application that uses a large number of regular
       expression patterns, it may be useful to store them in a precompiled
       form instead of having to compile them every time the application is
       run. However, if you are using the just-in-time optimization feature,
       it is not possible to save and reload the JIT data, because it is
       position-dependent. The host on which the patterns are reloaded must be
       running the same version of PCRE2, with the same code unit width, and
       must also have the same endianness, pointer width and PCRE2_SIZE type.
       For example, patterns compiled on a 32-bit system using PCRE2's 16-bit
       library cannot be reloaded on a 64-bit system, nor can they be reloaded
       using the 8-bit library.

       Note that "serialization" in PCRE2 does not convert compiled patterns
       to an abstract format like Java or .NET serialization. The serialized
       output is really just a bytecode dump, which is why it can only be
       reloaded in the same environment as the one that created it. Hence the
       restrictions mentioned above. Applications that are not statically
       linked with a fixed version of PCRE2 must be prepared to recompile
       patterns from their sources, in order to be immune to PCRE2 upgrades.


SECURITY CONCERNS
       The facility for saving and restoring compiled patterns is intended for
       use within individual applications. As such, the data supplied to
       pcre2_serialize_decode() is expected to be trusted data, not data from
       arbitrary external sources. There is only some simple consistency
       checking, not complete validation of what is being re-loaded. Corrupted
       data may cause undefined results. For example, if the length field of a
       pattern in the serialized data is corrupted, the deserializing code may
       read beyond the end of the byte stream that is passed to it.


SAVING COMPILED PATTERNS
       Before compiled patterns can be saved they must be serialized, which in
       PCRE2 means converting the pattern to a stream of bytes. A single byte
       stream may contain any number of compiled patterns, but they must all
       use the same character tables. A single copy of the tables is included
       in the byte stream (its size is 1088 bytes). For more details of
       character tables, see the section on locale support in the pcre2api
       documentation.

       The function pcre2_serialize_encode() creates a serialized byte stream
       from a list of compiled patterns. Its first two arguments specify the
       list, being a pointer to a vector of pointers to compiled patterns, and
       the length of the vector. The third and fourth arguments point to
       variables which are set to point to the created byte stream and its
       length, respectively. The final argument is a pointer to a general
       context, which can be used to specify custom memory management
       functions. If this argument is NULL, malloc() is used to obtain memory
       for the byte stream. The yield of the function is the number of
       serialized patterns, or one of the following negative error codes:

           PCRE2_ERROR_BADDATA     the number of patterns is zero or less
           PCRE2_ERROR_BADMAGIC    mismatch of id bytes in one of the patterns
           PCRE2_ERROR_NOMEMORY    memory allocation failed
           PCRE2_ERROR_MIXEDTABLES the patterns do not all use the same tables
           PCRE2_ERROR_NULL        The 1st, 3rd, or 4th argument is NULL

       PCRE2_ERROR_BADMAGIC means either that a pattern's code has been
       corrupted, or that a slot in the vector does not point to a compiled
       pattern.

       Once a set of patterns has been serialized you can save the data in any
       appropriate manner. Here is sample code that compiles two patterns and
       writes them to a file. It assumes that the variable fd refers to a file
       that is open for output. The error checking that should be present in a
       real application has been omitted for simplicity.

       int errorcode;
       uint8_t *bytes;
       PCRE2_SIZE erroroffset;
       PCRE2_SIZE bytescount;
       pcre2_code *list_of_codes[2];
       list_of_codes[0] = pcre2_compile("first pattern",
         PCRE2_ZERO_TERMINATED, 0, &errorcode, &erroroffset, NULL);
       list_of_codes[1] = pcre2_compile("second pattern",
         PCRE2_ZERO_TERMINATED, 0, &errorcode, &erroroffset, NULL);
       errorcode = pcre2_serialize_encode(list_of_codes, 2, &bytes,
         &bytescount, NULL);
       errorcode = fwrite(bytes, 1, bytescount, fd);

       Note that the serialized data is binary data that may contain any of
       the 256 possible byte values. On systems that make a distinction
       between binary and non-binary data, be sure that the file is opened for
       binary output.

       Serializing a set of patterns leaves the original data untouched, so
       they can still be used for matching. Their memory must eventually be
       freed in the usual way by calling pcre2_code_free(). When you have
       finished with the byte stream, it too must be freed by calling
       pcre2_serialize_free(). If this function is called with a NULL
       argument, it returns immediately without doing anything.


RE-USING PRECOMPILED PATTERNS
       In order to re-use a set of saved patterns you must first make the
       serialized byte stream available in main memory (for example, by
       reading from a file). The management of this memory block is up to the
       application. You can use the pcre2_serialize_get_number_of_codes()
       function to find out how many compiled patterns are in the serialized
       data without actually decoding the patterns:

       uint8_t *bytes = <serialized data>;
       int32_t number_of_codes = pcre2_serialize_get_number_of_codes(bytes);

       The pcre2_serialize_decode() function reads a byte stream and recreates
       the compiled patterns in new memory blocks, setting pointers to them in
       a vector. The first two arguments are a pointer to a suitable vector
       and its length, and the third argument points to a byte stream. The
       final argument is a pointer to a general context, which can be used to
       specify custom memory management functions for the decoded patterns. If
       this argument is NULL, malloc() and free() are used. After
       deserialization, the byte stream is no longer needed and can be
       discarded.

       pcre2_code *list_of_codes[2];
       uint8_t *bytes = <serialized data>;
       int32_t number_of_codes =
         pcre2_serialize_decode(list_of_codes, 2, bytes, NULL);

       If the vector is not large enough for all the patterns in the byte
       stream, it is filled with those that fit, and the remainder are
       ignored. The yield of the function is the number of decoded patterns,
       or one of the following negative error codes:

           PCRE2_ERROR_BADDATA     second argument is zero or less
           PCRE2_ERROR_BADMAGIC    mismatch of id bytes in the data
           PCRE2_ERROR_BADMODE     mismatch of code unit size or PCRE2 version
           PCRE2_ERROR_BADSERIALIZEDDATA
                                   other sanity check failure
           PCRE2_ERROR_MEMORY      memory allocation failed
           PCRE2_ERROR_NULL        first or third argument is NULL

       PCRE2_ERROR_BADMAGIC may mean that the data is corrupt, or that it was
       compiled on a system with different endianness.

       Decoded patterns can be used for matching in the usual way, and must be
       freed by calling pcre2_code_free(). However, be aware that there is a
       potential race issue if you are using multiple patterns that were
       decoded from a single byte stream in a multithreaded application. A
       single copy of the character tables is used by all the decoded patterns
       and a reference count is used to arrange for its memory to be
       automatically freed when the last pattern is freed, but there is no
       locking on this reference count. Therefore, if you want to call
       pcre2_code_free() for these patterns in different threads, you must
       arrange your own locking, and ensure that pcre2_code_free() cannot be
       called by two threads at the same time.

       If a pattern was processed by pcre2_jit_compile() before being
       serialized, the JIT data is discarded and so is no longer available
       after a save/restore cycle. You can, however, process a restored
       pattern with pcre2_jit_compile() if you wish.


AUTHOR
         Philip Hazel
         Retired from University Computing Service
         Cambridge, England.


REVISION
       Last updated: 19 January 2024
       Copyright (c) 1997-2018 University of Cambridge.


PCRE2 10.49-DEV                 19 January 2024              PCRE2SERIALIZE(3)
------------------------------------------------------------------------------


PCRE2SYNTAX(3)                       PCRE2                      PCRE2SYNTAX(3)


NAME
       pcre2syntax - PCRE2 syntax quick reference


PCRE2 REGULAR EXPRESSION SYNTAX SUMMARY
       The full syntax and semantics of the regular expression patterns that
       are supported by PCRE2 are described in the pcre2pattern documentation.
       This document contains a quick-reference summary of the pattern syntax
       followed by the syntax of replacement strings in substitution function.
       The full description of the latter is in the pcre2api documentation.


QUOTING
       \x         where x is non-alphanumeric is a literal x
       \Q...\E    treat enclosed characters as literal

       Note that white space inside \Q...\E is always treated as literal, even
       if PCRE2_EXTENDED is set, causing most other white space to be ignored.
       Note also that PCRE2's handling of \Q...\E has some differences from
       Perl's. See the pcre2pattern documentation for details.


BRACED ITEMS
       With one exception, wherever brace characters { and } are required to
       enclose data for constructions such as \g{2} or \k{name}, space and/or
       horizontal tab characters that follow { or precede } are allowed and
       are ignored. In the case of quantifiers, they may also appear before or
       after the comma. The exception is \u{...} which is not Perl-compatible
       and is recognized only when PCRE2_EXTRA_ALT_BSUX is set. This is an
       ECMAScript compatibility feature, and follows ECMAScript's behaviour.


ESCAPED CHARACTERS
       This table applies to ASCII and Unicode environments. An unrecognized
       escape sequence causes an error.

       \a         alarm, that is, the BEL character (hex 07)
       \cx        "control-x", where x is a non-control ASCII character
       \e         escape (hex 1B)
       \f         form feed (hex 0C)
       \n         newline (hex 0A)
       \r         carriage return (hex 0D)
       \t         tab (hex 09)
       \0dd       character with octal code 0dd
       \ddd       character with octal code ddd, or backreference
       \o{ddd..}  character with octal code ddd..
       \N{U+hh..} character with Unicode code point hh.. (Unicode mode only)
       \xhh       character with hex code hh
       \x{hh..}   character with hex code hh..

       \N{U+hh..} is synonymous with \x{hh..} but is not supported in
       environments that use EBCDIC code (mainly IBM mainframes). Note that \N
       not followed by an opening curly bracket has a different meaning (see
       below).

       If PCRE2_ALT_BSUX or PCRE2_EXTRA_ALT_BSUX is set ("ALT_BSUX mode"), the
       following are also recognized:

       \U         the character "U"
       \uhhhh     character with hex code hhhh
       \u{hh..}   character with hex code hh.. but only for EXTRA_ALT_BSUX

       When \x is not followed by {, one or two hexadecimal digits are read,
       but in ALT_BSUX mode \x must be followed by two hexadecimal digits to
       be recognized as a hexadecimal escape; otherwise it matches a literal
       "x". Likewise, if \u (in ALT_BSUX mode) is not followed by four
       hexadecimal digits or (in EXTRA_ALT_BSUX mode) a sequence of hex digits
       in curly brackets, it matches a literal "u".

       Note that \0dd is always an octal code. The treatment of backslash
       followed by a non-zero digit is complicated; for details see the
       section "Non-printing characters" in the pcre2pattern documentation,
       where details of escape processing in EBCDIC environments are also
       given.


CHARACTER TYPES
       .          any character except newline;
                    in dotall mode, any character whatsoever
       \C         one code unit, even in UTF mode (best avoided)
       \d         a decimal digit
       \D         a character that is not a decimal digit
       \h         a horizontal white space character
       \H         a character that is not a horizontal white space character
       \N         a character that is not a newline
       \p{_xx_}     a character with the _xx_ property
       \P{_xx_}     a character without the _xx_ property
       \R         a newline sequence
       \s         a white space character
       \S         a character that is not a white space character
       \v         a vertical white space character
       \V         a character that is not a vertical white space character
       \w         a "word" character
       \W         a "non-word" character
       \X         a Unicode extended grapheme cluster

       \C is dangerous because it may leave the current matching point in the
       middle of a UTF-8 or UTF-16 character. The application can lock out the
       use of \C by setting the PCRE2_NEVER_BACKSLASH_C option. It is also
       possible to build PCRE2 with the use of \C permanently disabled.

       By default, \d, \s, and \w match only ASCII characters, even in UTF-8
       mode or in the 16-bit and 32-bit libraries. However, if locale-specific
       matching is happening, \s and \w may also match characters with code
       points in the range 128-255. If the PCRE2_UCP option is set, the
       behaviour of these escape sequences is changed to use Unicode
       properties and they match many more characters, but there are some
       option settings that can restrict individual sequences to matching only
       ASCII characters.

       Property descriptions in \p and \P are matched caselessly; hyphens,
       underscores, and ASCII white space characters are ignored, in
       accordance with Unicode's "loose matching" rules. For example,
       \p{Bidi_Class=al} is the same as \p{ bidi class = AL }.


GENERAL CATEGORY PROPERTIES FOR \p AND \P
       C          Other
       Cc         Control
       Cf         Format
       Cn         Unassigned
       Co         Private use
       Cs         Surrogate

       L          Letter
       Lc         Cased letter, the union of Ll, Lu, and Lt
       L&         Synonym of Lc
       Ll         Lower case letter
       Lm         Modifier letter
       Lo         Other letter
       Lt         Title case letter
       Lu         Upper case letter

       M          Mark
       Mc         Spacing mark
       Me         Enclosing mark
       Mn         Non-spacing mark

       N          Number
       Nd         Decimal number
       Nl         Letter number
       No         Other number

       P          Punctuation
       Pc         Connector punctuation
       Pd         Dash punctuation
       Pe         Close punctuation
       Pf         Final punctuation
       Pi         Initial punctuation
       Po         Other punctuation
       Ps         Open punctuation

       S          Symbol
       Sc         Currency symbol
       Sk         Modifier symbol
       Sm         Mathematical symbol
       So         Other symbol

       Z          Separator
       Zl         Line separator
       Zp         Paragraph separator
       Zs         Space separator

       From release 10.45, when caseless matching is set, Ll, Lu, and Lt are
       all equivalent to Lc.


PCRE2 SPECIAL CATEGORY PROPERTIES FOR \p AND \P
       Xan        Alphanumeric: union of properties L and N
       Xps        POSIX space: property Z or tab, NL, VT, FF, CR
       Xsp        Perl space: property Z or tab, NL, VT, FF, CR
       Xuc        Universally-named character: one that can be
                    represented by a Universal Character Name
       Xwd        Perl word: property Xan or underscore

       Perl and POSIX space are now the same. Perl added VT to its space
       character set at release 5.18.


BINARY PROPERTIES FOR \p AND \P
       Unicode defines a number of binary properties, that is, properties
       whose only values are true or false. You can obtain a list of those
       that are recognized by \p and \P, along with their abbreviations, by
       running this command:

       pcre2test -LP


SCRIPT MATCHING WITH \p AND \P
       Many script names and their 4-letter abbreviations are recognized in
       \p{sc:...} or \p{scx:...} items, or on their own with \p (and also \P
       of course). You can obtain a list of these scripts by running this
       command:

       pcre2test -LS


THE BIDI_CLASS PROPERTY FOR \p AND \P
       \p{Bidi_Class:<class>}   matches a character with the given class
       \p{BC:<class>}           matches a character with the given class

       The recognized classes are:

       AL          Arabic letter
       AN          Arabic number
       B           paragraph separator
       BN          boundary neutral
       CS          common separator
       EN          European number
       ES          European separator
       ET          European terminator
       FSI         first strong isolate
       L           left-to-right
       LRE         left-to-right embedding
       LRI         left-to-right isolate
       LRO         left-to-right override
       NSM         non-spacing mark
       ON          other neutral
       PDF         pop directional format
       PDI         pop directional isolate
       R           right-to-left
       RLE         right-to-left embedding
       RLI         right-to-left isolate
       RLO         right-to-left override
       S           segment separator
       WS          white space


CHARACTER CLASSES
       [...]       positive character class
       [^...]      negative character class
       [x-y]       range (can be used for hex characters)
       [[:xxx:]]   positive POSIX named set
       [[:^xxx:]]  negative POSIX named set

       alnum       alphanumeric
       alpha       alphabetic
       ascii       0-127
       blank       space or tab
       cntrl       control character
       digit       decimal digit
       graph       printing, excluding space
       lower       lower case letter
       print       printing, including space
       punct       printing, excluding alphanumeric
       space       white space
       upper       upper case letter
       word        same as \w
       xdigit      hexadecimal digit

       In PCRE2, POSIX character set names recognize only ASCII characters by
       default, but some of them use Unicode properties if PCRE2_UCP is set.
       You can use \Q...\E inside a character class.

       When PCRE2_ALT_EXTENDED_CLASS is set, UTS#18 extended character classes
       may be used, allowing nested character classes, combined using set
       operators.

       [x&&[^y]]   UTS#18 extended character class

       x||y        set union (OR)
       x&&y        set intersection (AND)
       x--y        set difference (AND NOT)
       x~~y        set symmetric difference (XOR)


PERL EXTENDED CHARACTER CLASSES
       (?[...])                Perl extended character class
       (?[\p{Thai} & \p{Nd}])  operators; white space ignored
       (?[(x - y) & z])        parentheses for grouping

       (?[ [^3] & \p{Nd} ])    [...] is a nested ordinary class
       (?[ [:alpha:] - [z] ])  POSIX set is allowed outside [...]
       (?[ \d - [3] ])         backslash-escaped set is allowed outside [...]
       (?[ !\n & [:ascii:] ])  backslash-escaped character is allowed outside [...]
                           all other characters or ranges must be enclosed in [...]

       x|y, x+y                set union (OR)
       x&y                     set intersection (AND)
       x-y                     set difference (AND NOT)
       x^y                     set symmetric difference (XOR)
       !x                      set complement (NOT)

       Inside a Perl extended character class, [...] switches mode to be
       interpreted as an ordinary character class. Outside of a nested [...],
       the only items permitted are backslash-escapes, POSIX sets, operators,
       and parentheses. Inside a nested ordinary class, ^ has its usual
       meaning (inverts the class when used as the first character); outside
       of a nested class, ^ is the XOR operator.


QUANTIFIERS
       ?           0 or 1, greedy
       ?+          0 or 1, possessive
       ??          0 or 1, lazy
       *           0 or more, greedy
       *+          0 or more, possessive
       *?          0 or more, lazy
       +           1 or more, greedy
       ++          1 or more, possessive
       +?          1 or more, lazy
       {n}         exactly n
       {n,m}       at least n, no more than m, greedy
       {n,m}+      at least n, no more than m, possessive
       {n,m}?      at least n, no more than m, lazy
       {n,}        n or more, greedy
       {n,}+       n or more, possessive
       {n,}?       n or more, lazy
       {,m}        zero up to m, greedy
       {,m}+       zero up to m, possessive
       {,m}?       zero up to m, lazy


ANCHORS AND SIMPLE ASSERTIONS
       \b          word boundary
       \B          not a word boundary
       ^           start of subject
                     also after an internal newline in multiline mode
                     (after any newline if PCRE2_ALT_CIRCUMFLEX is set)
       \A          start of subject
       $           end of subject
                     also before newline at end of subject
                     also before internal newline in multiline mode
       \Z          end of subject
                     also before newline at end of subject
       \z          end of subject
       \G          first matching position in subject


REPORTED MATCH POINT SETTING
       \K          set reported start of match

       From release 10.38 \K is not permitted by default in lookaround
       assertions, for compatibility with Perl. However, if the
       PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK option is set, the previous behaviour
       is re-enabled. When this option is set, \K is honoured in positive
       assertions, but ignored in negative ones.


ALTERNATION
       expr|expr|expr...


CAPTURING
       (...)           capture group
       (?<name>...)    named capture group (Perl)
       (?'name'...)    named capture group (Perl)
       (?P<name>...)   named capture group (Python)
       (?:...)         non-capture group
       (?|...)         non-capture group; reset group numbers for
                        capture groups in each alternative

       In non-UTF modes, names may contain underscores and ASCII letters and
       digits; in UTF modes, any Unicode letters and Unicode decimal digits
       are permitted. In both cases, a name must not start with a digit.


ATOMIC GROUPS
       (?>...)         atomic non-capture group
       (*atomic:...)   atomic non-capture group


COMMENT
       (?#....)        comment (not nestable)


OPTION SETTING
       Changes of these options within a group are automatically cancelled at
       the end of the group.

       (?a)            all ASCII options
       (?aD)           restrict \d to ASCII in UCP mode
       (?aS)           restrict \s to ASCII in UCP mode
       (?aW)           restrict \w to ASCII in UCP mode
       (?aP)           restrict all POSIX classes to ASCII in UCP mode
       (?aT)           restrict POSIX digit classes to ASCII in UCP mode
       (?i)            caseless
       (?J)            allow duplicate named groups
       (?m)            multiline
       (?n)            no auto capture
       (?r)            restrict caseless to either ASCII or non-ASCII
       (?s)            single line (dotall)
       (?U)            default ungreedy (lazy)
       (?x)            ignore white space except in classes or \Q...\E
       (?xx)           as (?x) but also ignore space and tab in classes
       (?-...)         unset the given option(s)
       (?^)            unset imnrsx options

       (?aP) implies (?aT) as well, though this has no additional effect.
       However, it means that (?-aP) also implies (?-aT) and disables all
       ASCII restrictions for POSIX classes.

       Unsetting x or xx unsets both. Several options may be set at once, and
       a mixture of setting and unsetting such as (?i-x) is allowed, but there
       may be only one hyphen. Setting (but no unsetting) is allowed after (?^
       for example (?^in). An option setting may appear at the start of a
       non-capture group, for example (?i:...).

       The following are recognized only at the very start of a pattern or
       after one of the newline or \R sequences or options with similar
       syntax. More than one of them may appear. For the first three, d is a
       decimal number.

       (*LIMIT_DEPTH=d)     set the backtracking limit to d
       (*LIMIT_HEAP=d)      set the heap size limit to d * 1024 bytes
       (*LIMIT_MATCH=d)     set the match limit to d
       (*CASELESS_RESTRICT) set PCRE2_EXTRA_CASELESS_RESTRICT when matching
       (*NOTEMPTY)          set PCRE2_NOTEMPTY when matching
       (*NOTEMPTY_ATSTART)  set PCRE2_NOTEMPTY_ATSTART when matching
       (*NO_AUTO_POSSESS)   no auto-possessification (PCRE2_NO_AUTO_POSSESS)
       (*NO_DOTSTAR_ANCHOR) no .* anchoring (PCRE2_NO_DOTSTAR_ANCHOR)
       (*NO_JIT)            disable JIT optimization
       (*NO_START_OPT)      no start-match optimization (PCRE2_NO_START_OPTIMIZE)
       (*TURKISH_CASING)    set PCRE2_EXTRA_TURKISH_CASING when matching
       (*UTF)               set appropriate UTF mode for the library in use
       (*UCP)               set PCRE2_UCP (use Unicode properties for \d etc)

       Note that LIMIT_DEPTH, LIMIT_HEAP, and LIMIT_MATCH can only reduce the
       value of the limits set by the caller of pcre2_match() or
       pcre2_dfa_match(), not increase them. LIMIT_RECURSION is an obsolete
       synonym for LIMIT_DEPTH. The application can lock out the use of (*UTF)
       and (*UCP) by setting the PCRE2_NEVER_UTF or PCRE2_NEVER_UCP options,
       respectively, at compile time.


NEWLINE CONVENTION
       These are recognized only at the very start of the pattern or after
       option settings with a similar syntax.

       (*CR)           carriage return only
       (*LF)           linefeed only
       (*CRLF)         carriage return followed by linefeed
       (*ANYCRLF)      all three of the above
       (*ANY)          any Unicode newline sequence
       (*NUL)          the NUL character (binary zero)


WHAT \R MATCHES
       These are recognized only at the very start of the pattern or after
       option setting with a similar syntax.

       (*BSR_ANYCRLF)  CR, LF, or CRLF
       (*BSR_UNICODE)  any Unicode newline sequence


LOOKAHEAD AND LOOKBEHIND ASSERTIONS
       (?=...)                     )
       (*pla:...)                  ) positive lookahead
       (*positive_lookahead:...)   )

       (?!...)                     )
       (*nla:...)                  ) negative lookahead
       (*negative_lookahead:...)   )

       (?<=...)                    )
       (*plb:...)                  ) positive lookbehind
       (*positive_lookbehind:...)  )

       (?<!...)                    )
       (*nlb:...)                  ) negative lookbehind
       (*negative_lookbehind:...)  )

       Each top-level branch of a lookbehind must have a limit for the number
       of characters it matches. If any branch can match a variable number of
       characters, the maximum for each branch is limited to a value set by
       the caller of pcre2_compile() or defaulted. The default is set when
       PCRE2 is built (ultimate default 255). If every branch matches a fixed
       number of characters, the limit for each branch is 65535 characters.


NON-ATOMIC LOOKAROUND ASSERTIONS
       These assertions are specific to PCRE2 and are not Perl-compatible.

       (?*...)                                )
       (*napla:...)                           ) synonyms
       (*non_atomic_positive_lookahead:...)   )

       (?<*...)                               )
       (*naplb:...)                           ) synonyms
       (*non_atomic_positive_lookbehind:...)  )


SUBSTRING SCAN ASSERTION
       This feature is not Perl-compatible.

       (*scan_substring:(grouplist)...)  scan captured substring
       (*scs:(grouplist)...)             scan captured substring

       The comma-separated list "grouplist" may identify groups in any of the
       following ways:

       n       absolute reference
       +n      relative reference
       -n      relative reference
       <name>  name
       'name'  name


SCRIPT RUNS
       (*script_run:...)           ) script run, can be backtracked into
       (*sr:...)                   )

       (*atomic_script_run:...)    ) atomic script run
       (*asr:...)                  )


BACKREFERENCES
       \n              reference by number (can be ambiguous)
       \gn             reference by number
       \g{n}           reference by number
       \g+n            relative reference by number (PCRE2 extension)
       \g-n            relative reference by number
       \g{+n}          relative reference by number (PCRE2 extension)
       \g{-n}          relative reference by number
       \k<name>        reference by name (Perl)
       \k'name'        reference by name (Perl)
       \g{name}        reference by name (Perl)
       \k{name}        reference by name (.NET)
       (?P=name)       reference by name (Python)


SUBROUTINE REFERENCES (POSSIBLY RECURSIVE)
       (?R)            recurse whole pattern
       (?n)            call subroutine by absolute number
       (?+n)           call subroutine by relative number
       (?-n)           call subroutine by relative number
       (?&name)        call subroutine by name (Perl)
       (?P>name)       call subroutine by name (Python)
       \g<name>        call subroutine by name (Oniguruma)
       \g'name'        call subroutine by name (Oniguruma)
       \g<n>           call subroutine by absolute number (Oniguruma)
       \g'n'           call subroutine by absolute number (Oniguruma)
       \g<+n>          call subroutine by relative number (PCRE2 extension)
       \g'+n'          call subroutine by relative number (PCRE2 extension)
       \g<-n>          call subroutine by relative number (PCRE2 extension)
       \g'-n'          call subroutine by relative number (PCRE2 extension)

       The variants using parentheses (?...) may also specify a list of
       capture groups to return, which shall be retained in the calling
       subexpression if set during the recursion (this feature is not
       supported by Perl).

       (?R(grouplist))       recurse whole pattern, returning capture groups
                               (PCRE2 extension)
       (?n(grouplist))       )
       (?+n(grouplist))      ) call subroutine, returning capture groups
       (?-n(grouplist))      )   (PCRE2 extension)
       (?&name(grouplist))   )
       (?P>name(grouplist))  )

       The comma-separated list "grouplist" uses the same syntax as
       (*scan_substring:(grouplist)...), and may identify groups in any of the
       following ways:

       n       absolute reference
       +n      relative reference
       -n      relative reference
       <name>  name
       'name'  name


CONDITIONAL PATTERNS
       (?(condition)yes-pattern)
       (?(condition)yes-pattern|no-pattern)

       (?(n)                absolute reference condition
       (?(+n)               relative reference condition (PCRE2 extension)
       (?(-n)               relative reference condition (PCRE2 extension)
       (?(<name>)           named reference condition (Perl)
       (?('name')           named reference condition (Perl)
       (?(name)             named reference condition (PCRE2, deprecated)
       (?(R)                overall recursion condition
       (?(Rn)               specific numbered group recursion condition
       (?(R&name)           specific named group recursion condition
       (?(DEFINE)           define groups for reference
       (?(VERSION[>]=n[.m]) test PCRE2 version
       (?(assert)           assertion condition

       Note the ambiguity of (?(R) and (?(Rn) which might be named reference
       conditions or recursion tests. Such a condition is interpreted as a
       reference condition if the relevant named group exists.

       The parts within brackets for the VERSION conditional syntax could be
       ommited. The fractional part of the version number defaults to 0 in
       that case.


BACKTRACKING CONTROL
       All backtracking control verbs may be in the form (*VERB:NAME). For
       (*MARK) the name is mandatory, for the others it is optional. (*SKIP)
       changes its behaviour if :NAME is present. The others just set a name
       for passing back to the caller, but this is not a name that (*SKIP) can
       see. The following act immediately they are reached:

       (*ACCEPT)       force successful match
       (*FAIL)         force backtrack; synonym (*F)
       (*MARK:NAME)    set name to be passed back; synonym (*:NAME)

       The following act only when a subsequent match failure causes a
       backtrack to reach them. They all force a match failure, but they
       differ in what happens afterwards. Those that advance the
       start-of-match point do so only if the pattern is not anchored.

       (*COMMIT)       overall failure, no advance of starting point
       (*PRUNE)        advance to next starting character
       (*SKIP)         advance to current matching position
       (*SKIP:NAME)    advance to position corresponding to an earlier
                       (*MARK:NAME); if not found, the (*SKIP) is ignored
       (*THEN)         local failure, backtrack to next alternation

       The effect of one of these verbs in a group called as a subroutine is
       confined to the subroutine call.


CALLOUTS
       (?C)            callout (assumed number 0)
       (?Cn)           callout with numerical data n
       (?C"text")      callout with string data

       The allowed string delimiters are ` ' " ^ % # $ (which are the same for
       the start and the end), and the starting delimiter { matched with the
       ending delimiter }. To encode the ending delimiter within the string,
       double it.


REPLACEMENT STRINGS
       If the PCRE2_SUBSTITUTE_LITERAL option is set, a replacement string for
       pcre2_substitute() is not interpreted. Otherwise, by default, the only
       special character is the dollar character in one of the following
       forms:

       $$                  insert a dollar character
       $n or ${n}          insert the contents of group _n_
       $<name>             insert the contents of named group
       $0 or $&            insert the entire matched substring
       $`                  insert the substring that precedes the match
       $'                  insert the substring that follows the match
       $_                  insert the entire input string
       $+                  insert the highest-numbered capture group which matched
       $*MARK or ${*MARK}  insert a control verb name

       For ${n}, n can be a name or a number. If PCRE2_SUBSTITUTE_EXTENDED is
       set, there is additional interpretation:

        1. Backslash is an escape character, and the forms described in
           "ESCAPED CHARACTERS" above are recognized. Also:

       \Q...\E can be used to suppress interpretation
       \l      force the next character to lower case
       \u      force the next character to upper case
       \L      force subsequent characters to lower case
       \U      force subsequent characters to upper case
       \u\L    force next character to upper case, then all lower
       \l\U    force next character to lower case, then all upper
       \E      end \L or \U case forcing
       \b      backspace character (note: as in character class in pattern)
       \v      vertical tab character (note: not the same as in a pattern)

        1. The Python form \g<n>, where the angle brackets are part of the
           syntax and n is either a group name or a number, is recognized as
           an alternative way of inserting the contents of a group, for
           example \g<3>.

        2. Capture substitution supports the following additional forms:

       ${n:-string}             default for unset group
       ${n:+string1:string2}    values for set/unset group

       The substitution strings themselves are expanded. Backslash can be used
       to escape colons and closing curly brackets.


SEE ALSO
       pcre2pattern(3), pcre2api(3), pcre2callout(3), pcre2matching(3),
       pcre2(3).


AUTHOR
         Philip Hazel
         Retired from University Computing Service
         Cambridge, England.


REVISION
       Last updated: 14 October 2025
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.49-DEV                 14 October 2025                 PCRE2SYNTAX(3)
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PCRE2UNICODE(3)                      PCRE2                     PCRE2UNICODE(3)


NAME
       pcre2unicode - PCRE2 Unicode support


UNICODE AND UTF SUPPORT
       PCRE2 is normally built with Unicode support, though if you do not need
       it, you can build it without, in which case the library will be
       smaller. With Unicode support, PCRE2 has knowledge of Unicode character
       properties and can process strings of text in UTF-8, UTF-16, and UTF-32
       format (depending on the code unit width), but this is not the default.
       Unless specifically requested, PCRE2 treats each code unit in a string
       as one character.

       There are two ways of telling PCRE2 to switch to UTF mode, where
       characters may consist of more than one code unit and the range of
       values is constrained. The program can call pcre2_compile() with the
       PCRE2_UTF option, or the pattern may start with the sequence (*UTF).
       However, the latter facility can be locked out by the PCRE2_NEVER_UTF
       option. That is, the programmer can prevent the supplier of the pattern
       from switching to UTF mode.

       Note that the PCRE2_MATCH_INVALID_UTF option (see below) forces
       PCRE2_UTF to be set.

       In UTF mode, both the pattern and any subject strings that are matched
       against it are treated as UTF strings instead of strings of individual
       one-code-unit characters. There are also some other changes to the way
       characters are handled, as documented below.


UNICODE PROPERTY SUPPORT
       When PCRE2 is built with Unicode support, the escape sequences \p{..},
       \P{..}, and \X can be used. This is not dependent on the PCRE2_UTF
       setting. The Unicode properties that can be tested are a subset of
       those that Perl supports. Currently they are limited to the general
       category properties such as Lu for an upper case letter or Nd for a
       decimal number, the derived properties Any and Lc (synonym L&), the
       Unicode script names such as Arabic or Han, Bidi_Class, Bidi_Control,
       and a few binary properties.

       The full lists are given in the pcre2pattern and pcre2syntax
       documentation. In general, only the short names for properties are
       supported. For example, \p{L} matches a letter. Its longer synonym,
       \p{Letter}, is not supported. Furthermore, in Perl, many properties may
       optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE2
       does not support this.


WIDE CHARACTERS AND UTF MODES
       Code points less than 256 can be specified in patterns by either braced
       or unbraced hexadecimal escape sequences (for example, \x{b3} or \xb3).
       Larger values have to use braced sequences. Unbraced octal code points
       up to \777 are also recognized; larger ones can be coded using \o{...}.

       The escape sequence \N{U+<hex digits>} is recognized as another way of
       specifying a Unicode character by code point in a UTF mode. It is not
       allowed in non-UTF mode.

       In UTF mode, repeat quantifiers apply to complete UTF characters, not
       to individual code units.

       In UTF mode, the dot metacharacter matches one UTF character instead of
       a single code unit.

       In UTF mode, capture group names are not restricted to ASCII, and may
       contain any Unicode letters and decimal digits, as well as underscore.

       The escape sequence \C can be used to match a single code unit in UTF
       mode, but its use can lead to some strange effects because it breaks up
       multi-unit characters (see the description of \C in the pcre2pattern
       documentation). For this reason, there is a build-time option that
       disables support for \C completely. There is also a less draconian
       compile-time option for locking out the use of \C when a pattern is
       compiled.

       The use of \C is not supported by the alternative matching function
       pcre2_dfa_match() when in UTF-8 or UTF-16 mode, that is, when a
       character may consist of more than one code unit. The use of \C in
       these modes provokes a match-time error. Also, the JIT optimization
       does not support \C in these modes. If JIT optimization is requested
       for a UTF-8 or UTF-16 pattern that contains \C, it will not succeed,
       and so when pcre2_match() is called, the matching will be carried out
       by the interpretive function.

       The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly test
       characters of any code value, but, by default, the characters that
       PCRE2 recognizes as digits, spaces, or word characters remain the same
       set as in non-UTF mode, all with code points less than 256. This
       remains true even when PCRE2 is built to include Unicode support,
       because to do otherwise would slow down matching in many common cases.
       Note that this also applies to \b and \B, because they are defined in
       terms of \w and \W. If you want to test for a wider sense of, say,
       "digit", you can use explicit Unicode property tests such as \p{Nd}.
       Alternatively, if you set the PCRE2_UCP option, the way that the
       character escapes work is changed so that Unicode properties are used
       to determine which characters match, though there are some options that
       suppress this for individual escapes. For details see the section on
       generic character types in the pcre2pattern documentation.

       Like the escapes, characters that match the POSIX named character
       classes are all low-valued characters unless the PCRE2_UCP option is
       set, but there is an option to override this.

       In contrast to the character escapes and character classes, the special
       horizontal and vertical white space escapes (\h, \H, \v, and \V) do
       match all the appropriate Unicode characters, whether or not PCRE2_UCP
       is set.


UNICODE CASE-EQUIVALENCE
       If either PCRE2_UTF or PCRE2_UCP is set, upper/lower case processing
       makes use of Unicode properties except for characters whose code points
       are less than 128 and that have at most two case-equivalent values. For
       these, a direct table lookup is used for speed. A few Unicode
       characters such as Greek sigma have more than two code points that are
       case-equivalent, and these are treated specially. Setting PCRE2_UCP
       without PCRE2_UTF allows Unicode-style case processing for non-UTF
       character encodings such as UCS-2.

       There are two ASCII characters (S and K) that, in addition to their
       ASCII lower case equivalents, have a non-ASCII one as well (long S and
       Kelvin sign). Recognition of these non-ASCII characters as
       case-equivalent to their ASCII counterparts can be disabled by setting
       the PCRE2_EXTRA_CASELESS_RESTRICT option. When this is set, all
       characters in a case equivalence must either be ASCII or non-ASCII;
       there can be no mixing.

         Without PCRE2_EXTRA_CASELESS_RESTRICT:
           'k' = 'K' = U+212A (Kelvin sign)
           's' = 'S' = U+017F (long S)
         With PCRE2_EXTRA_CASELESS_RESTRICT:
           'k' = 'K'
           U+212A (Kelvin sign)  only case-equivalent to itself
           's' = 'S'
           U+017F (long S)       only case-equivalent to itself

       One language family, Turkish and Azeri, has its own case-insensitivity
       rules, which can be selected by setting PCRE2_EXTRA_TURKISH_CASING.
       This alters the behaviour of the 'i', 'I', U+0130 (capital I with dot
       above), and U+0131 (small dotless i) characters.

         Without PCRE2_EXTRA_TURKISH_CASING:
           'i' = 'I'
           U+0130 (capital I with dot above)  only case-equivalent to itself
           U+0131 (small dotless i)           only case-equivalent to itself
         With PCRE2_EXTRA_TURKISH_CASING:
           'i' = U+0130 (capital I with dot above)
           U+0131 (small dotless i) = 'I'

       It is not allowed to specify both PCRE2_EXTRA_CASELESS_RESTRICT and
       PCRE2_EXTRA_TURKISH_CASING together.

       From release 10.45 the Unicode letter properties Lu (upper case), Ll
       (lower case), and Lt (title case) are all treated as Lc (cased letter)
       when caseless matching is set by the PCRE2_CASELESS option or (?i)
       within the pattern.


SCRIPT RUNS
       The pattern constructs (*script_run:...) and (*atomic_script_run:...),
       with synonyms (*sr:...) and (*asr:...), verify that the string matched
       within the parentheses is a script run. In concept, a script run is a
       sequence of characters that are all from the same Unicode script.
       However, because some scripts are commonly used together, and because
       some diacritical and other marks are used with multiple scripts, it is
       not that simple.

       Every Unicode character has a Script property, mostly with a value
       corresponding to the name of a script, such as Latin, Greek, or
       Cyrillic. There are also three special values:

       "Unknown" is used for code points that have not been assigned, and also
       for the surrogate code points. In the PCRE2 32-bit library, characters
       whose code points are greater than the Unicode maximum (U+10FFFF),
       which are accessible only in non-UTF mode, are assigned the Unknown
       script.

       "Common" is used for characters that are used with many scripts. These
       include punctuation, emoji, mathematical, musical, and currency
       symbols, and the ASCII digits 0 to 9.

       "Inherited" is used for characters such as diacritical marks that
       modify a previous character. These are considered to take on the script
       of the character that they modify.

       Some Inherited characters are used with many scripts, but many of them
       are only normally used with a small number of scripts. For example,
       U+102E0 (Coptic Epact thousands mark) is used only with Arabic and
       Coptic. In order to make it possible to check this, a Unicode property
       called Script Extension exists. Its value is a list of scripts that
       apply to the character. For the majority of characters, the list
       contains just one script, the same one as the Script property. However,
       for characters such as U+102E0 more than one Script is listed. There
       are also some Common characters that have a single, non-Common script
       in their Script Extension list.

       The next section describes the basic rules for deciding whether a given
       string of characters is a script run. Note, however, that there are
       some special cases involving the Chinese Han script, and an additional
       constraint for decimal digits. These are covered in subsequent
       sections.

   Basic script run rules
       A string that is less than two characters long is a script run. This is
       the only case in which an Unknown character can be part of a script
       run. Longer strings are checked using only the Script Extensions
       property, not the basic Script property.

       If a character's Script Extension property is the single value
       "Inherited", it is always accepted as part of a script run. This is
       also true for the property "Common", subject to the checking of decimal
       digits described below. All the remaining characters in a script run
       must have at least one script in common in their Script Extension
       lists. In set-theoretic terminology, the intersection of all the sets
       of scripts must not be empty.

       A simple example is an Internet name such as "google.com". The letters
       are all in the Latin script, and the dot is Common, so this string is a
       script run. However, the Cyrillic letter "o" looks exactly the same as
       the Latin "o"; a string that looks the same, but with Cyrillic "o"s is
       not a script run.

       More interesting examples involve characters with more than one script
       in their Script Extension. Consider the following characters:

         U+060C  Arabic comma
         U+06D4  Arabic full stop

       The first has the Script Extension list Arabic, Hanifi Rohingya,
       Syriac, and Thaana; the second has just Arabic and Hanifi Rohingya.
       Both of them could appear in script runs of either Arabic or Hanifi
       Rohingya. The first could also appear in Syriac or Thaana script runs,
       but the second could not.

   The Chinese Han script
       The Chinese Han script is commonly used in conjunction with other
       scripts for writing certain languages. Japanese uses the Hiragana and
       Katakana scripts together with Han; Korean uses Hangul and Han;
       Taiwanese Mandarin uses Bopomofo and Han. These three combinations are
       treated as special cases when checking script runs and are, in effect,
       "virtual scripts". Thus, a script run may contain a mixture of
       Hiragana, Katakana, and Han, or a mixture of Hangul and Han, or a
       mixture of Bopomofo and Han, but not, for example, a mixture of Hangul
       and Bopomofo and Han. PCRE2 (like Perl) follows Unicode's Technical
       Standard 39 ("Unicode Security Mechanisms",
       http://unicode.org/reports/tr39/) in allowing such mixtures.

   Decimal digits
       Unicode contains many sets of 10 decimal digits in different scripts,
       and some scripts (including the Common script) contain more than one
       set. Some of these decimal digits them are visually indistinguishable
       from the common ASCII digits. In addition to the script checking
       described above, if a script run contains any decimal digits, they must
       all come from the same set of 10 adjacent characters.


VALIDITY OF UTF STRINGS
       When the PCRE2_UTF option is set, the strings passed as patterns and
       subjects are (by default) checked for validity on entry to the relevant
       functions. If an invalid UTF string is passed, a negative error code is
       returned. The code unit offset to the offending character can be
       extracted from the match data block by calling pcre2_get_startchar(),
       which is used for this purpose after a UTF error.

       In some situations, you may already know that your strings are valid,
       and therefore want to skip these checks in order to improve
       performance, for example in the case of a long subject string that is
       being scanned repeatedly. If you set the PCRE2_NO_UTF_CHECK option at
       compile time or at match time, PCRE2 assumes that the pattern or
       subject it is given (respectively) contains only valid UTF code unit
       sequences.

       If you pass an invalid UTF string when PCRE2_NO_UTF_CHECK is set, the
       result is undefined and your program may crash or loop indefinitely or
       give incorrect results. There is, however, one mode of matching that
       can handle invalid UTF subject strings. This is enabled by passing
       PCRE2_MATCH_INVALID_UTF to pcre2_compile() and is discussed below in
       the next section. The rest of this section covers the case when
       PCRE2_MATCH_INVALID_UTF is not set.

       Passing PCRE2_NO_UTF_CHECK to pcre2_compile() just disables the UTF
       check for the pattern; it does not also apply to subject strings. If
       you want to disable the check for a subject string you must pass this
       same option to pcre2_match() or pcre2_dfa_match().

       UTF-16 and UTF-32 strings can indicate their endianness by special code
       knows as a byte-order mark (BOM). The PCRE2 functions do not handle
       this, expecting strings to be in host byte order.

       Unless PCRE2_NO_UTF_CHECK is set, a UTF string is checked before any
       other processing takes place. In the case of pcre2_match() and
       pcre2_dfa_match() calls with a non-zero starting offset, the check is
       applied only to that part of the subject that could be inspected during
       matching, and there is a check that the starting offset points to the
       first code unit of a character or to the end of the subject. If there
       are no lookbehind assertions in the pattern, the check starts at the
       starting offset. Otherwise, it starts at the length of the longest
       lookbehind before the starting offset, or at the start of the subject
       if there are not that many characters before the starting offset. Note
       that the sequences \b and \B are one-character lookbehinds.

       In addition to checking the format of the string, there is a check to
       ensure that all code points lie in the range U+0 to U+10FFFF, excluding
       the surrogate area. The so-called "non-character" code points are not
       excluded because Unicode corrigendum #9 makes it clear that they should
       not be.

       Characters in the "Surrogate Area" of Unicode are reserved for use by
       UTF-16, where they are used in pairs to encode code points with values
       greater than 0xFFFF. The code points that are encoded by UTF-16 pairs
       are available independently in the UTF-8 and UTF-32 encodings. (In
       other words, the whole surrogate thing is a fudge for UTF-16 which
       unfortunately messes up UTF-8 and UTF-32.)

       Setting PCRE2_NO_UTF_CHECK at compile time does not disable the error
       that is given if an escape sequence for an invalid Unicode code point
       is encountered in the pattern. If you want to allow escape sequences
       such as \x{d800} (a surrogate code point) you can set the
       PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES extra option. However, this is
       possible only in UTF-8 and UTF-32 modes, because these values are not
       representable in UTF-16.

   Errors in UTF-8 strings
       The following negative error codes are given for invalid UTF-8 strings:

       PCRE2_ERROR_UTF8_ERR1, PCRE2_ERROR_UTF8_ERR2, PCRE2_ERROR_UTF8_ERR3,
       PCRE2_ERROR_UTF8_ERR4, PCRE2_ERROR_UTF8_ERR5:: The string ends with a
       truncated UTF-8 character; the code specifies how many bytes are
       missing (1 to 5). Although RFC 3629 restricts UTF-8 characters to be no
       longer than 4 bytes, the encoding scheme (originally defined by RFC
       2279) allows for up to 6 bytes, and this is checked first; hence the
       possibility of 4 or 5 missing bytes.

       PCRE2_ERROR_UTF8_ERR6, PCRE2_ERROR_UTF8_ERR7, PCRE2_ERROR_UTF8_ERR8,
       PCRE2_ERROR_UTF8_ERR9, PCRE2_ERROR_UTF8_ERR10:: The two most
       significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of the
       character do not have the binary value 0b10 (that is, either the most
       significant bit is 0, or the next bit is 1).

       PCRE2_ERROR_UTF8_ERR11, PCRE2_ERROR_UTF8_ERR12:: A character that is
       valid by the RFC 2279 rules is either 5 or 6 bytes long; these code
       points are excluded by RFC 3629.

       PCRE2_ERROR_UTF8_ERR13 A 4-byte character has a value greater than
                              0x10ffff; these code points are excluded by RFC
                              3629.

       PCRE2_ERROR_UTF8_ERR14 A 3-byte character has a value in the range
                              0xd800 to 0xdfff; this range of code points are
                              reserved by RFC 3629 for use with UTF-16, and so
                              are excluded from UTF-8.

       PCRE2_ERROR_UTF8_ERR15, PCRE2_ERROR_UTF8_ERR16, PCRE2_ERROR_UTF8_ERR17,
       PCRE2_ERROR_UTF8_ERR18, PCRE2_ERROR_UTF8_ERR19:: A 2-, 3-, 4-, 5-, or
       6-byte character is "overlong", that is, it codes for a value that can
       be represented by fewer bytes, which is invalid. For example, the two
       bytes 0xc0, 0xae give the value 0x2e, whose correct coding uses just
       one byte.

       PCRE2_ERROR_UTF8_ERR20 The two most significant bits of the first byte
                              of a character have the binary value 0b10 (that
                              is, the most significant bit is 1 and the second
                              is 0). Such a byte can only validly occur as the
                              second or subsequent byte of a multi-byte
                              character.

       PCRE2_ERROR_UTF8_ERR21 The first byte of a character has the value 0xfe
                              or 0xff. These values can never occur in a valid
                              UTF-8 string.

   Errors in UTF-16 strings
       The following negative error codes are given for invalid UTF-16
       strings:

           PCRE2_ERROR_UTF16_ERR1 Missing low surrogate at end of string
           PCRE2_ERROR_UTF16_ERR2 Invalid low surrogate follows high surrogate
           PCRE2_ERROR_UTF16_ERR3 Isolated low surrogate

   Errors in UTF-32 strings
       The following negative error codes are given for invalid UTF-32
       strings:

           PCRE2_ERROR_UTF32_ERR1 Surrogate character (0xd800 to 0xdfff)
           PCRE2_ERROR_UTF32_ERR2 Code point is greater than 0x10ffff


MATCHING IN INVALID UTF STRINGS
       You can run pattern matches on subject strings that may contain invalid
       UTF sequences if you call pcre2_compile() with the
       PCRE2_MATCH_INVALID_UTF option. This is supported by pcre2_match(),
       including JIT matching, but not by pcre2_dfa_match(). When
       PCRE2_MATCH_INVALID_UTF is set, it forces PCRE2_UTF to be set as well.
       Note, however, that the pattern itself must be a valid UTF string.

       If you do not set PCRE2_MATCH_INVALID_UTF when calling pcre2_compile(),
       and you are not certain that your subject strings are valid UTF
       sequences, you should not make use of the JIT "fast path" function
       pcre2_jit_match() because it bypasses sanity checks, including the one
       for UTF validity. An invalid string may cause undefined behaviour,
       including looping, crashing, or giving the wrong answer.

       Setting PCRE2_MATCH_INVALID_UTF does not affect what pcre2_compile()
       generates, but if pcre2_jit_compile() is subsequently called, it does
       generate different code. If JIT is not used, the option affects the
       behaviour of the interpretive code in pcre2_match(). When
       PCRE2_MATCH_INVALID_UTF is set at compile time, PCRE2_NO_UTF_CHECK is
       ignored at match time.

       In this mode, an invalid code unit sequence in the subject never
       matches any pattern item. It does not match dot, it does not match
       \p{Any}, it does not even match negative items such as [^X]. A
       lookbehind assertion fails if it encounters an invalid sequence while
       moving the current point backwards. In other words, an invalid UTF code
       unit sequence acts as a barrier which no match can cross.

       You can also think of this as the subject being split up into fragments
       of valid UTF, delimited internally by invalid code unit sequences. The
       pattern is matched fragment by fragment. The result of a successful
       match, however, is given as code unit offsets in the entire subject
       string in the usual way. There are a few points to consider:

       The internal boundaries are not interpreted as the beginnings or ends
       of lines and so do not match circumflex or dollar characters in the
       pattern.

       If pcre2_match() is called with an offset that points to an invalid
       UTF-sequence, that sequence is skipped, and the match starts at the
       next valid UTF character, or the end of the subject.

       At internal fragment boundaries, \b and \B behave in the same way as at
       the beginning and end of the subject. For example, a sequence such as
       \bWORD\b would match an instance of WORD that is surrounded by invalid
       UTF code units.

       Using PCRE2_MATCH_INVALID_UTF, an application can run matches on
       arbitrary data, knowing that any matched strings that are returned are
       valid UTF. This can be useful when searching for UTF text in executable
       or other binary files.

       Note, however, that the 16-bit and 32-bit PCRE2 libraries process
       strings as sequences of uint16_t or uint32_t code points. They cannot
       find valid UTF sequences within an arbitrary string of bytes unless
       such sequences are suitably aligned.


AUTHOR
         Philip Hazel
         Retired from University Computing Service
         Cambridge, England.


REVISION
       Last updated: 27 November 2024
       Copyright (c) 1997-2024 University of Cambridge.


PCRE2 10.49-DEV                27 November 2024                PCRE2UNICODE(3)
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