regex(3) — Linux manual page


regex(3)                Library Functions Manual                regex(3)

NAME         top

       regcomp, regexec, regerror, regfree - POSIX regex functions

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <regex.h>

       int regcomp(regex_t *restrict preg, const char *restrict regex,
                   int cflags);
       int regexec(const regex_t *restrict preg, const char *restrict string,
                   size_t nmatch, regmatch_t pmatch[restrict .nmatch],
                   int eflags);

       size_t regerror(int errcode, const regex_t *restrict preg,
                   char errbuf[restrict .errbuf_size], 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 is 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 used.

              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

              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 is the bitwise-or of zero or more of the following flags:

              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

   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 <regex.h>.

           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 terminating 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, regcomp().

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        │

STANDARDS         top


HISTORY         top


EXAMPLES         top

       #include <stdint.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <regex.h>

       #define ARRAY_SIZE(arr) (sizeof((arr)) / sizeof((arr)[0]))

       static const char *const str =
               "1) John Driverhacker;\n2) John Doe;\n3) John Foo;\n";
       static const char *const re = "John.*o";

       int main(void)
           static const char *s = str;
           regex_t     regex;
           regmatch_t  pmatch[1];
           regoff_t    off, len;

           if (regcomp(&regex, re, REG_NEWLINE))

           printf("String = \"%s\"\n", str);

           for (unsigned int i = 0; ; i++) {
               if (regexec(&regex, s, ARRAY_SIZE(pmatch), pmatch, 0))

               off = pmatch[0].rm_so + (s - str);
               len = pmatch[0].rm_eo - pmatch[0].rm_so;
               printf("#%zu:\n", i);
               printf("offset = %jd; length = %jd\n", (intmax_t) off,
                       (intmax_t) len);
               printf("substring = \"%.*s\"\n", len, s + pmatch[0].rm_so);

               s += pmatch[0].rm_eo;


SEE ALSO         top

       grep(1), regex(7)

       The glibc manual section, Regular Expressions

Linux man-pages 6.04           2023-03-30                       regex(3)

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