pcre2posix(3) — Linux manual page


PCRE2POSIX(3)           Library Functions Manual           PCRE2POSIX(3)

NAME         top

       PCRE2 - Perl-compatible regular expressions (revised API)

SYNOPSIS         top

       #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         top

       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.


       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.


       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:


       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.


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


       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).


       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
       that REG_NOSPEC is not part of the POSIX standard.


       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.


       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


       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 matchine \d, \w, etc., instead of just
       recognizing ASCII values. Note that REG_UCP is not part of the
       POSIX standard.


       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.


       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

       In the absence of these flags, no options are passed to the
       native function.  This means the 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

       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.


       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

                                 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


       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:


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


       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.


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


       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         top

       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         top

       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         top

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.

REVISION         top

       Last updated: 14 November 2023
       Copyright (c) 1997-2023 University of Cambridge.

COLOPHON         top

       This page is part of the PCRE (Perl Compatible Regular
       Expressions) project.  Information about the project can be found
       at ⟨http://www.pcre.org/⟩.  If you have a bug report for this
       manual page, see
       ⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩.  This page was
       obtained from the tarball fetched from
       ⟨https://github.com/PhilipHazel/pcre2.git⟩ on 2023-12-22.  If you
       discover any rendering problems in this HTML version of the page,
       or you believe there is a better or more up-to-date source for
       the page, or you have corrections or improvements to the
       information in this COLOPHON (which is not part of the original
       manual page), send a mail to man-pages@man7.org

PCRE2 10.43                 14 November 2023               PCRE2POSIX(3)

Pages that refer to this page: pcre2api(3)