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pcre2posix(3)              Library Functions Manual              pcre2posix(3)


NAME

       PCRE2 - Perl-compatible regular expressions (revised 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 NOTE: 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 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, regex_t for compiled internal forms, and 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 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 REG_PEND below). The preg argument is a pointer to
       a 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 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:

         REG_DOTALL

       The PCRE2_DOTALL option is set when the regular expression is passed
       for compilation to the native function. Note that REG_DOTALL is not
       part of the POSIX standard.

         REG_ICASE

       The PCRE2_CASELESS option is set when the regular expression is passed
       for compilation to the native function.

         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 REG_NEWLINE (see the following
       section).

         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 REG_NOSPEC are
       REG_ICASE, REG_NOSUB, REG_PEND, and REG_UTF. Note that REG_NOSPEC is
       not part of the POSIX standard.

         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 PCRE
       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.

         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 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.

         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 REG_UCP is not part of the POSIX standard.

         REG_UNGREEDY

       The PCRE2_UNGREEDY option is set when the regular expression is passed
       for compilation to the native function. Note that REG_UNGREEDY is not
       part of the POSIX standard.

         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 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 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 REG_NEWLINE
       action. When using the POSIX API, passing REG_NEWLINE to PCRE2's
       pcre2_regcomp() function causes PCRE2_MULTILINE to be passed to
       pcre2_compile(), and 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 REG_STARTEND below), subject to the options in eflags.  These
       can be:

         REG_NOTBOL

       The PCRE2_NOTBOL option is set when calling the underlying PCRE2
       matching function.

         REG_NOTEMPTY

       The PCRE2_NOTEMPTY option is set when calling the underlying PCRE2
       matching function. Note that REG_NOTEMPTY is not part of the POSIX
       standard. However, setting this option can give more POSIX-like
       behaviour in some situations.

         REG_NOTEOL

       The PCRE2_NOTEOL option is set when calling the underlying PCRE2
       matching function.

         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 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 REG_NOTBOL; REG_STARTEND affects only the location and
       length of the string, not how it is matched. Setting REG_STARTEND and
       passing pmatch as NULL are mutually exclusive; the error REG_INVARG is
       returned.

       If the pattern was compiled with the 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
       REG_STARTEND).

       The value of nmatch may be zero, and the value pmatch may be NULL
       (unless 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 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.

       regmatch_t as well as the 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 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: 19 January 2024
       Copyright (c) 1997-2024 University of Cambridge.

PCRE2 10.43                     19 January 2024                  pcre2posix(3)

pcre2 10.43 - Generated Sat Mar 2 12:19:05 CST 2024
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