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);
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.
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.
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.
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').
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.1-2001, POSIX.1-2008.
#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