Regfree (3) Linux Manual Page

regcomp, regexec, regerror, regfree – POSIX regex functions

Synopsis

#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);

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

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
([ha...])

not containing a newline does not match a newline.

Match-beginning-of-line operator
(ha)

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().

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 failure.

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 attributes(7).

Interface	Attribute	Value
regcomp(), regexec()	Thread safety	MT-Safe locale
regerror()	Thread safety	MT-Safe env
regfree()	Thread safety	MT-Safe

Conforming To

POSIX.1-2001, POSIX.1-2008.

Examples

#include <regex.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.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(&regex, re, REG_NEWLINE))
        exit(EXIT_FAILURE);
    printf("String = \"%s\"\n", str);
    printf("Matches:\n");
    for (int i = 0;; i++) {
        if (regexec(&regex, 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);
}

Colophon

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