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[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
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 argu-
ments 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 immedi-
ately before a newline, regardless of whether eflags contains
REG_NOTEOL.
POSIX regex matching
regexec() is used to match a null-terminated string against the precom-
piled pattern buffer, preg. nmatch and pmatch are used to provide in-
formation regarding the location of any matches. eflags is the bit-
wise-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 di-
mensioned to have at least nmatch elements. These are filled in by
regexec() with substring match addresses. The offsets of the subex-
pression 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 el-
ement 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 er-
rbuf_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
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.
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.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.
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(7).
┌─────────────────────────────────────┬───────────────┬────────────────┐
│Interface │ Attribute │ Value │
├─────────────────────────────────────┼───────────────┼────────────────┤
│regcomp(), regexec() │ Thread safety │ MT-Safe locale │
├─────────────────────────────────────┼───────────────┼────────────────┤
│regerror() │ Thread safety │ MT-Safe env │
├─────────────────────────────────────┼───────────────┼────────────────┤
│regfree() │ Thread safety │ MT-Safe │
└─────────────────────────────────────┴───────────────┴────────────────┘
STANDARDS
POSIX.1-2001, POSIX.1-2008.
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 6.03 2023-02-05 regex(3)
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