PCREPOSIX(3)            (09 January 2012)            PCREPOSIX(3)

     NAME
          PCRE - Perl-compatible regular expressions.

     SYNOPSIS

          #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 docu-
          mentation for a description of PCRE's native API, which con-
          tains much additional functionality. There is no POSIX-style
          wrapper for PCRE's 16-bit and 32-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 applica-
          tion 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, sub-
          ject to the setting of various PCRE options, as described

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          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.

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            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 oth-
          erwise. The preg structure is filled in on success, and one
          member of the structure is public: re_nsub contains the num-
          ber of capturing subpatterns in the regular expression. Var-
          ious 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 differ-
          ent 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 possi-
          bilities for matching newline characters in PCRE:

                                    Default   Change with

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            . 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 set-
          ting 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

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          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 cau-
          tion 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 sub-
          string and the offset to the first character after the end
          of each substring, respectively. The 0th element of the vec-
          tor relates to the entire portion of string that was
          matched; subsequent elements relate to the capturing subpat-
          terns 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 reg-
          free() frees all such memory, after which preg may no longer
          be used as a compiled expression.

     AUTHOR

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

     REVISION

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

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