PCREPOSIX(3)              OpenBSD Programmer's Manual             PCREPOSIX(3)



NAME
       PCRE - Perl-compatible regular expressions.

SYNOPSIS OF POSIX API

       #include <pcreposix.h>

       int regcomp(regex_t *preg, const char *pattern,
            int cflags);

       int regexec(regex_t *preg, const char *string,
            size_t nmatch, regmatch_t pmatch[], int eflags);

       size_t regerror(int errcode, const regex_t *preg,
            char *errbuf, size_t errbuf_size);

       void regfree(regex_t *preg);

DESCRIPTION

       This set of functions provides a POSIX-style API for the PCRE regular
       expression 8-bit library. See the pcreapi documentation for a
       description of PCRE's native API, which contains much additional
       functionality. There is no POSIX-style wrapper for PCRE's 16-bit
       library.

       The functions described here are just wrapper functions that ultimately
       call the PCRE native API. Their prototypes are defined in the
       pcreposix.h header file, and on Unix systems the library itself is
       called pcreposix.a, so can be accessed by adding -lpcreposix to the
       command for linking an application that uses them. Because the POSIX
       functions call the native ones, it is also necessary to add -lpcre.

       I have implemented only those POSIX option bits that can be reasonably
       mapped to PCRE native options. 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 PCRE as a replacement library. Other POSIX options
       are not even defined.

       There are also some other options that are not defined by POSIX. These
       have been added at the request of users who want to make use of certain
       PCRE-specific features via the POSIX calling interface.

       When PCRE 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 PCRE 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-byte encoding domains it is
       probably even less compatible.

       The header for these functions is supplied as pcreposix.h to avoid any
       potential clash with other POSIX libraries. It can, of course, be
       renamed or aliased as regex.h, which is the "correct" name. 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.

COMPILING A PATTERN

       The function regcomp() is called to compile a pattern into an internal
       form. The pattern is a C string terminated by a binary zero, and is
       passed in the argument pattern. 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.

       The argument cflags is either zero, or contains one or more of the bits
       defined by the following macros:

         REG_DOTALL

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

         REG_NEWLINE

       The PCRE_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_NOSUB

       The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is
       passed for compilation to the native function. In addition, when a
       pattern that is compiled with this flag is passed to regexec() for
       matching, the nmatch and pmatch arguments are ignored, and no captured
       strings are returned.

         REG_UCP

       The PCRE_UCP option is set when the regular expression is passed for
       compilation to the native function. This causes PCRE to use Unicode
       properties when matchine \d, \w, etc., instead of just recognizing
       ASCII values. Note that REG_UTF8 is not part of the POSIX standard.

         REG_UNGREEDY

       The PCRE_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_UTF8

       The PCRE_UTF8 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_UTF8 is not part of the POSIX standard.

       In the absence of these flags, no options are passed to the native
       function.  This means the the regex is compiled with PCRE 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
       PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE.
       It does not affect the way newlines are matched by . (they are not) or
       by a negative class such as [^a] (they are).

       The yield of regcomp() is zero on success, and non-zero otherwise. The
       preg structure is filled in on success, and one member of the structure
       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 regcomp() is non-zero, you must not attempt to
       use the contents of the preg structure. If, for example, you pass it to
       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 PCRE to obey POSIX semantics, but
       then PCRE was never intended to be a POSIX engine. The following table
       lists the different possibilities for matching newline characters in
       PCRE:

                                 Default   Change with

         . matches newline          no     PCRE_DOTALL
         newline matches [^a]       yes    not changeable
         $ matches \n at end        yes    PCRE_DOLLARENDONLY
         $ matches \n in middle     no     PCRE_MULTILINE
         ^ matches \n in middle     no     PCRE_MULTILINE

       This is the equivalent table for POSIX:

                                 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

       PCRE's behaviour is the same as Perl's, except that there is no
       equivalent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl,
       there is no way to stop newline from matching [^a].

       The default POSIX newline handling can be obtained by setting
       PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE
       behave exactly as for the REG_NEWLINE action.

MATCHING A PATTERN

       The function 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 PCRE_NOTBOL option is set when calling the underlying PCRE matching
       function.

         REG_NOTEMPTY

       The PCRE_NOTEMPTY option is set when calling the underlying PCRE
       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 PCRE_NOTEOL option is set when calling the underlying PCRE matching
       function.

         REG_STARTEND

       The string is considered to start at string + pmatch[0].rm_so and to
       have a terminating NUL located at string + pmatch[0].rm_eo (there need
       not actually be a NUL at that location), regardless of the value of
       nmatch. 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
       of the string, not how it is matched.

       If the pattern was compiled with the REG_NOSUB flag, no data about any
       matched strings is returned. The nmatch and pmatch arguments of
       regexec() are ignored.

       If the value of nmatch is zero, or if the value pmatch is NULL, 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 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.

       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 regerror() function maps a non-zero errorcode from either regcomp()
       or 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. The length of the message,
       including the zero, is limited to errbuf_size. The yield of the
       function is the size of buffer needed to hold the whole message.

MEMORY USAGE

       Compiling a regular expression causes memory to be allocated and
       associated with the preg structure. The function regfree() frees all
       such memory, after which preg may no longer be used as a compiled
       expression.

AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION

       Last updated: 09 January 2012
       Copyright (c) 1997-2012 University of Cambridge.



                                                                  PCREPOSIX(3)
