regcomp(3) — Linux manual page


REGEX(3)                  Linux Programmer's Manual                 REGEX(3)

NAME         top

       regcomp, regexec, regerror, regfree - POSIX regex functions

SYNOPSIS         top

       #include <sys/types.h>
       #include <regex.h>

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

       int regexec(const 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         top

   POSIX regex compiling
       regcomp() is used to compile a regular expression into a form that is
       suitable for subsequent regexec() searches.

       regcomp() is supplied with preg, a pointer to a pattern buffer
       storage area; regex, a pointer to the null-terminated string and
       cflags, flags used to determine the type of compilation.

       All regular expression searching must be done via a compiled pattern
       buffer, thus regexec() must always be supplied with the address of a
       regcomp() initialized pattern buffer.

       cflags may be the bitwise-or of zero or more of the following:

              Use POSIX Extended Regular Expression syntax when interpreting
              regex.  If not set, POSIX Basic Regular Expression syntax is

              Do not differentiate case.  Subsequent regexec() searches
              using this pattern buffer will be case insensitive.

              Do not report position of matches.  The nmatch and pmatch
              arguments to regexec() are ignored if the pattern buffer
              supplied was compiled with this flag set.

              Match-any-character operators don't match a newline.

              A nonmatching list ([^...])  not containing a newline does not
              match a newline.

              Match-beginning-of-line operator (^) matches the empty string
              immediately after a newline, regardless of whether eflags, the
              execution flags of regexec(), contains REG_NOTBOL.

              Match-end-of-line operator ($) matches the empty string
              immediately before a newline, regardless of whether eflags
              contains REG_NOTEOL.

   POSIX regex matching
       regexec() is used to match a null-terminated string against the
       precompiled pattern buffer, preg.  nmatch and pmatch are used to
       provide information regarding the location of any matches.  eflags
       may be the bitwise-or of one or both of REG_NOTBOL and REG_NOTEOL
       which cause changes in matching behavior described below.

              The match-beginning-of-line operator always fails to match
              (but see the compilation flag REG_NEWLINE above).  This flag
              may be used when different portions of a string are passed to
              regexec() and the beginning of the string should not be
              interpreted as the beginning of the line.

              The match-end-of-line operator always fails to match (but see
              the compilation flag REG_NEWLINE above).

              Use pmatch[0] on the input string, starting at byte
              pmatch[0].rm_so and ending before byte pmatch[0].rm_eo.  This
              allows matching embedded NUL bytes and avoids a strlen(3) on
              large strings.  It does not use nmatch on input, and does not
              change REG_NOTBOL or REG_NEWLINE processing.  This flag is a
              BSD extension, not present in POSIX.

   Byte offsets
       Unless REG_NOSUB was set for the compilation of the pattern buffer,
       it is possible to obtain match addressing information.  pmatch must
       be dimensioned to have at least nmatch elements.  These are filled in
       by regexec() with substring match addresses.  The offsets of the
       subexpression starting at the ith open parenthesis are stored in
       pmatch[i].  The entire regular expression's match addresses are
       stored in pmatch[0].  (Note that to return the offsets of N
       subexpression matches, nmatch must be at least N+1.)  Any unused
       structure elements will contain the value -1.

       The regmatch_t structure which is the type of pmatch is defined in

           typedef struct {
               regoff_t rm_so;
               regoff_t rm_eo;
           } regmatch_t;

       Each rm_so element that is not -1 indicates the start offset of the
       next largest substring match within the string.  The relative rm_eo
       element indicates the end offset of the match, which is the offset of
       the first character after the matching text.

   POSIX error reporting
       regerror() is used to turn the error codes that can be returned by
       both regcomp() and regexec() into error message strings.

       regerror() is passed the error code, errcode, the pattern buffer,
       preg, a pointer to a character string buffer, errbuf, and the size of
       the string buffer, errbuf_size.  It returns the size of the errbuf
       required to contain the null-terminated error message string.  If
       both errbuf and errbuf_size are nonzero, errbuf is filled in with the
       first errbuf_size - 1 characters of the error message and a terminat‐
       ing null byte ('\0').

   POSIX pattern buffer freeing
       Supplying regfree() with a precompiled pattern buffer, preg will free
       the memory allocated to the pattern buffer by the compiling process,

RETURN VALUE         top

       regcomp() returns zero for a successful compilation or an error code
       for failure.

       regexec() returns zero for a successful match or REG_NOMATCH for

ERRORS         top

       The following errors can be returned by regcomp():

              Invalid use of back reference operator.

              Invalid use of pattern operators such as group or list.

              Invalid use of repetition operators such as using '*' as the
              first character.

              Un-matched brace interval operators.

              Un-matched bracket list operators.

              Invalid collating element.

              Unknown character class name.

              Nonspecific error.  This is not defined by POSIX.2.

              Trailing backslash.

              Un-matched parenthesis group operators.

              Invalid use of the range operator; for example, the ending
              point of the range occurs prior to the starting point.

              Compiled regular expression requires a pattern buffer larger
              than 64 kB.  This is not defined by POSIX.2.

              The regex routines ran out of memory.

              Invalid back reference to a subexpression.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface            Attribute     Value          │
       │regcomp(), regexec() │ Thread safety │ MT-Safe locale │
       │regerror()           │ Thread safety │ MT-Safe env    │
       │regfree()            │ Thread safety │ MT-Safe        │

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008.

SEE ALSO         top

       grep(1), regex(7)

       The glibc manual section, Regular Expressions

COLOPHON         top

       This page is part of release 5.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

GNU                              2020-08-13                         REGEX(3)

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