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regex(3) — Linux manual page

NAME \|LIBRARY \|SYNOPSIS \|DESCRIPTION \|RETURN VALUE \|ERRORS \|ATTRIBUTES \|STANDARDS \|HISTORY \|CAVEATS \|EXAMPLES \|SEE ALSO \|COLOPHON

regex(3)                 Library Functions Manualregex(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 *restrictpreg, const char *restrictregex,intcflags);int regexec(const regex_t *restrictpreg, const char *restrictstring,size_tnmatch, regmatch_tpmatch[_Nullable restrictnmatch],inteflags);size_t regerror(size_t errbuf_size;interrcode, const regex_t *_Nullable restrictpreg,charerrbuf[_Nullable restricterrbuf_size],size_terrbuf_size);void regfree(regex_t *preg);typedef struct {size_t    re_nsub;} regex_t;typedef struct {regoff_t  rm_so;regoff_t  rm_eo;} regmatch_t;typedef/* ... */regoff_t;

DESCRIPTION top

Compilationregcomp() is used to compile a regular expression into a form that       is suitable for subsequentregexec() searches.       On success, the pattern buffer at*preg is initialized.regex is       a null-terminated string.  The locale must be the same when       runningregexec().       Afterregcomp() succeeds,preg->re_nsub holds the number of       subexpressions inregex.  Thus, a value ofpreg->re_nsub + 1       passed asnmatch toregexec() is sufficient to capture all       matches.cflags is the bitwise OR of zero or more of the following:REG_EXTENDED              Use POSIX Extended Regular Expression syntax when              interpretingregex.  If not set, POSIX Basic Regular              Expression syntax is used.REG_ICASE              Do not differentiate case.  Subsequentregexec() searches              using this pattern buffer will be case insensitive.REG_NOSUB              Report only overall success.regexec() will use onlypmatch forREG_STARTEND, ignoringnmatch.REG_NEWLINE              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 whethereflags, the execution flags ofregexec(), containsREG_NOTBOL.              Match-end-of-line operator ($) matches the empty string              immediately before a newline, regardless of whethereflags              containsREG_NOTEOL.Matchingregexec() is used to match a null-terminated string against the       compiled pattern buffer in*preg, which must have been initialised       withregcomp().eflags is the bitwise OR of zero or more of the       following flags:REG_NOTBOL              The match-beginning-of-line operator always fails to match              (but see the compilation flagREG_NEWLINEabove).  This              flag may be used when different portions of a string are              passed toregexec() and the beginning of the string should              not be interpreted as the beginning of the line.REG_NOTEOL              The match-end-of-line operator always fails to match (but              see the compilation flagREG_NEWLINEabove).REG_STARTEND              Match [string + pmatch[0].rm_so,string + pmatch[0].rm_eo)              instead of [string,string + strlen(string)).  This allows              matching embedded NUL bytes and avoids astrlen(3) on              known-length strings.  If any matches are returned              (REG_NOSUBwasn't passed toregcomp(), the match succeeded,              andnmatch > 0), they overwritepmatch as usual, and the              match offsets remain relative tostring (notstring +pmatch[0].rm_so).  This flag is a BSD extension, not              present in POSIX.Match offsets       UnlessREG_NOSUBwas passed toregcomp(), it is possible to obtain       the locations of matches withinstring:regexec() fillsnmatch       elements ofpmatch with results:pmatch[0] corresponds to the       entire match,pmatch[1] to the first subexpression, etc.  If there       were more matches thannmatch, they are discarded; if fewer,       unused elements ofpmatch are filled with-1s.       Each returned valid (non--1) match corresponds to the range       [string + rm_so,string + rm_eo).regoff_t is a signed integer type capable of storing the largest       value that can be stored in either anptrdiff_t type or assize_t       type.Error reportingregerror() is used to turn the error codes that can be returned by       bothregcomp() andregexec() into error message strings.       Ifpreg isn't a null pointer,errcode must be the latest error       returned from an operation onpreg.       Iferrbuf_size isn't 0, up toerrbuf_size bytes are copied toerrbuf; the error string is always null-terminated, and truncated       to fit.Freeingregfree() deinitializes the pattern buffer at*preg, freeing any       associated memory;*preg must have been initialized viaregcomp().

RETURN VALUE top

regcomp() returns zero for a successful compilation or an error       code for failure.regexec() returns zero for a successful match orREG_NOMATCHfor       failure.regerror() returns the size of the buffer required to hold the       string.

ERRORS top

       The following errors can be returned byregcomp():REG_BADBR              Invalid use of back reference operator.REG_BADPAT              Invalid use of pattern operators such as group or list.REG_BADRPT              Invalid use of repetition operators such as using '*' as              the first character.REG_EBRACE              Un-matched brace interval operators.REG_EBRACK              Un-matched bracket list operators.REG_ECOLLATE              Invalid collating element.REG_ECTYPE              Unknown character class name.REG_EEND              Nonspecific error.  This is not defined by POSIX.REG_EESCAPE              Trailing backslash.REG_EPAREN              Un-matched parenthesis group operators.REG_ERANGE              Invalid use of the range operator; for example, the ending              point of the range occurs prior to the starting point.REG_ESIZE              Compiled regular expression requires a pattern buffer              larger than 64 kB.  This is not defined by POSIX.REG_ESPACE              The regex routines ran out of memory.REG_ESUBREG              Invalid back reference to a subexpression.

ATTRIBUTES top

       For an explanation of the terms used in this section, seeattributes(7).       ┌───────────────────────────────┬───────────────┬────────────────┐       │Interface│Attribute│Value│       ├───────────────────────────────┼───────────────┼────────────────┤       │regcomp(),regexec()          │ Thread safety │ MT-Safe locale │       ├───────────────────────────────┼───────────────┼────────────────┤       │regerror()                    │ Thread safety │ MT-Safe env    │       ├───────────────────────────────┼───────────────┼────────────────┤       │regfree()                     │ Thread safety │ MT-Safe        │       └───────────────────────────────┴───────────────┴────────────────┘

STANDARDS top

       POSIX.1-2008.

HISTORY top

       POSIX.1-2001.       Prior to POSIX.1-2008,regoff_t was required to be capable of       storing the largest value that can be stored in either anoff_t       type or assize_t type.

CAVEATS top

re_nsub is only required to be initialized ifREG_NOSUBwasn't       specified, but all known implementations initialize it regardless.       Bothregex_t andregmatch_t may (and do) have more members, in any       order.  Always reference them by name.

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))               exit(EXIT_FAILURE);           printf("String = \"%s\"\n", str);           printf("Matches:\n");           for (unsigned int i = 0; ; i++) {               if (regexec(&regex, s, ARRAY_SIZE(pmatch), pmatch, 0))                   break;               off = pmatch[0].rm_so + (s - str);               len = pmatch[0].rm_eo - pmatch[0].rm_so;               printf("#%u:\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;           }           exit(EXIT_SUCCESS);       }

COLOPHON top

       This page is part of theman-pages (Linux kernel and C library       user-space interface documentation) project.  Information about       the project can be found at        ⟨https://www.kernel.org/doc/man-pages/⟩.  If you have a bug report       for this manual page, see       ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.       This page was obtained from the tarball man-pages-6.15.tar.gz       fetched from       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on       2025-08-11.  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 isnot       part of the original manual page), send a mail to       man-pages@man7.orgLinux man-pages 6.15            2025-06-28regex(3)

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