regex(3) Library Functions Manual regex(3)
NAME
regcomp, regexec, regerror, regfree - POSIX regex functions
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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[_Nullable restrict .nmatch],
int eflags);
size_t regerror(int errcode, const regex_t *_Nullable restrict preg,
char errbuf[_Nullable restrict .errbuf_size],
size_t errbuf_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
Compilation
regcomp() is used to compile a regular expression into a form that is
suitable for subsequent regexec() searches.
On success, the pattern buffer at *preg is initialized. regex is a
null-terminated string. The locale must be the same when running
regexec().
After regcomp() succeeds, preg->re_nsub holds the number of subexpres-
sions in regex. Thus, a value of preg->re_nsub + 1 passed as nmatch to
regexec() 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 interpreting
regex. If not set, POSIX Basic Regular Expression syntax is
used.
REG_ICASE
Do not differentiate case. Subsequent regexec() searches using
this pattern buffer will be case insensitive.
REG_NOSUB
Report only overall success. regexec() will use only pmatch for
REG_STARTEND, ignoring nmatch.
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 im-
mediately after a newline, regardless of whether eflags, the exe-
cution flags of regexec(), contains REG_NOTBOL.
Match-end-of-line operator ($) matches the empty string immedi-
ately before a newline, regardless of whether eflags contains
REG_NOTEOL.
Matching
regexec() is used to match a null-terminated string against the compiled
pattern buffer in *preg, which must have been initialised with
regexec(). 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 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.
REG_NOTEOL
The match-end-of-line operator always fails to match (but see the
compilation flag REG_NEWLINE above).
REG_STARTEND
Match [string + pmatch[0].rm_so, string + pmatch[0].rm_eo) in-
stead of [string, string + strlen(string)). This allows matching
embedded NUL bytes and avoids a strlen(3) on known-length
strings. If any matches are returned (REG_NOSUB wasn't passed to
regcomp(), the match succeeded, and nmatch > 0), they overwrite
pmatch as usual, and the match offsets remain relative to string
(not string + pmatch[0].rm_so). This flag is a BSD extension,
not present in POSIX.
Match offsets
Unless REG_NOSUB was passed to regcomp(), it is possible to obtain the
locations of matches within string: regexec() fills nmatch elements of
pmatch with results: pmatch[0] corresponds to the entire match,
pmatch[1] to the first subexpression, etc. If there were more matches
than nmatch, they are discarded; if fewer, unused elements of pmatch 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 an ptrdiff_t type or a ssize_t type.
Error reporting
regerror() is used to turn the error codes that can be returned by both
regcomp() and regexec() into error message strings.
If preg isn't a null pointer, errcode must be the latest error returned
from an operation on preg.
If errbuf_size isn't 0, up to errbuf_size bytes are copied to errbuf;
the error string is always null-terminated, and truncated to fit.
Freeing
regfree() deinitializes the pattern buffer at *preg, freeing any associ-
ated memory; *preg must have been initialized via regcomp().
RETURN VALUE
regcomp() returns zero for a successful compilation or an error code for
failure.
regexec() returns zero for a successful match or REG_NOMATCH for fail-
ure.
regerror() returns the size of the buffer required to hold the string.
ERRORS
The following errors can be returned by regcomp():
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
For an explanation of the terms used in this section, see attributes(7).
┌─────────────────────────────────────┬───────────────┬────────────────┐
│ Interface │ Attribute │ Value │
├─────────────────────────────────────┼───────────────┼────────────────┤
│ regcomp(), regexec() │ Thread safety │ MT-Safe locale │
├─────────────────────────────────────┼───────────────┼────────────────┤
│ regerror() │ Thread safety │ MT-Safe env │
├─────────────────────────────────────┼───────────────┼────────────────┤
│ regfree() │ Thread safety │ MT-Safe │
└─────────────────────────────────────┴───────────────┴────────────────┘
STANDARDS
POSIX.1-2008.
HISTORY
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 an off_t type or a
ssize_t type.
CAVEATS
re_nsub is only required to be initialized if REG_NOSUB wasn't speci-
fied, but all known implementations initialize it regardless.
Both regex_t and regmatch_t may (and do) have more members, in any or-
der. Always reference them by name.
EXAMPLES
#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(®ex, re, REG_NEWLINE))
exit(EXIT_FAILURE);
printf("String = \"%s\"\n", str);
printf("Matches:\n");
for (unsigned int i = 0; ; i++) {
if (regexec(®ex, s, ARRAY_SIZE(pmatch), pmatch, 0))
break;
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;
}
exit(EXIT_SUCCESS);
}
SEE ALSO
grep(1), regex(7)
The glibc manual section, Regular Expressions
Linux man-pages 6.9.1 2024-06-15 regex(3)
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