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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | EXAMPLES | SEE ALSO |
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regex(3) Library Functions Manual regex(3)
regcomp, regexec, regerror, regfree - POSIX regex functions
Standard C library (libc, -lc)
#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);
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:
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
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.
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 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 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
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 <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().
regcomp() returns zero for a successful compilation or an error
code for failure.
regexec() returns zero for a successful match or REG_NOMATCH for
failure.
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.2.
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.2.
REG_ESPACE
The regex routines ran out of memory.
REG_ESUBREG
Invalid back reference to a subexpression.
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 │
└───────────────────────────────┴───────────────┴────────────────┘
POSIX.1-2008.
POSIX.1-2001.
#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);
}
grep(1), regex(7)
The glibc manual section, Regular Expressions
Linux man-pages 6.04 2023-03-30 regex(3)
Pages that refer to this page: bash(1), killall(1), pmdamailq(1), pmdaweblog(1), pmie(1), pmval(1), trace-cmd-list(1), trace-cmd-report(1), ausearch_add_regex(3), nl_langinfo(3), pmregisterderived(3), re_comp(3), regex_t(3type), rpmatch(3), sysconf(3), tracefs_event_systems(3), regex(7)
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HTML rendering created 2023-06-24 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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