regcomp, regexec, regerror, regfree - POSIX regex functions
#include <regex.h>
int regcomp(regex_t *preg, const char *regex, int cflags);
int regexec(const regex_t *preg, const char *string, size_t nmatch,
regmatch_t pmatch[], int eflags);
size_t regerror(int errcode, const regex_t *preg, char *errbuf,
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:
Use POSIX Extended Regular Expression syntax when
interpreting regex
. If not set, POSIX Basic
Regular Expression syntax is used.
Do not differentiate case. Subsequent regexec() searches using this pattern buffer will be case insensitive.
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.
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:
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.
The match-end-of-line operator always fails to match (but see the compilation flag REG_NEWLINE above).
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 i
th 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().
#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 (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("#%d:\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);
}
The following errors can be returned by regcomp():
Invalid use of back reference operator.
Invalid use of pattern operators such as group or list.
Invalid use of repetition operators such as using '*' as the first character.
Un-matched brace interval operators.
Un-matched bracket list operators.
Invalid collating element.
Unknown character class name.
Nonspecific error. This is not defined by POSIX.2.
Trailing backslash.
Un-matched parenthesis group operators.
Invalid use of the range operator; for example, the ending point of the range occurs prior to the starting point.
Compiled regular expression requires a pattern buffer larger than 64 kB. This is not defined by POSIX.2.
The regex routines ran out of memory.
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.
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