regex

Section: C Library Functions (3)
Updated: 2023-10-31
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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[_Nullable restrict .nmatch],
            int eflags);

size_t regerror(int errcode, const regex_t *_Nullable restrict preg,
            char errbuf[_Nullable restrict .errbuf_size],
            size_t errbuf_size);
void regfree(regex_t *preg);

typedef struct {
    size_t    re_nsub;
} regex_t;

typedef struct {
    regoff_t  rm_so;
    regoff_t  rm_eo;
} regmatch_t;

typedef /* ... */  regoff_t;

DESCRIPTION

Compilation

regcomp() is used to compile a regular expression into a form that is suitable for subsequent regexec() searches.

On success, the pattern buffer at *preg is initialized. regex is a null-terminated string. The locale must be the same when running regexec().

After regcomp() succeeds, preg->re_nsub holds the number of subexpressions in regex. Thus, a value of preg->re_nsub + 1 passed as nmatch to regexec() is sufficient to capture all matches.

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: Report only overall success. regexec() will use only pmatch for REG_STARTEND, ignoring nmatch.
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.

Matching

regexec() is used to match a null-terminated string against the compiled pattern buffer in *preg, which must have been initialised with regexec(). 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: Match [string + pmatch[0].rm_so, string + pmatch[0].rm_eo) instead of [string, string + strlen(string)). This allows matching embedded NUL bytes and avoids a strlen(3) on known-length strings. If any matches are returned (REG_NOSUB wasn't passed to regcomp(), the match succeeded, and nmatch > 0), they overwrite pmatch as usual, and the match offsets remain relative to string (not string + pmatch[0].rm_so). This flag is a BSD extension, not present in POSIX.

Match offsets

Unless REG_NOSUB was passed to regcomp(), it is possible to obtain the locations of matches within string: regexec() fills nmatch elements of pmatch with results: pmatch[0] corresponds to the entire match, pmatch[1] to the first subexpression, etc. If there were more matches than nmatch, they are discarded; if fewer, unused elements of pmatch are filled with -1s.

Each returned valid (non--1) match corresponds to the range [string + rm_so, string + rm_eo).

regoff_t is a signed integer type capable of storing the largest value that can be stored in either an ptrdiff_t type or a ssize_t type.

Error reporting

regerror() is used to turn the error codes that can be returned by both regcomp() and regexec() into error message strings.

If preg isn't a null pointer, errcode must be the latest error returned from an operation on preg.

If errbuf_size isn't 0, up to errbuf_size bytes are copied to errbuf; the error string is always null-terminated, and truncated to fit.

Freeing

regfree() deinitializes the pattern buffer at *preg, freeing any associated memory; *preg must have been initialized via 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.

regerror() returns the size of the buffer required to hold the string.

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.
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.
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

STANDARDS

POSIX.1-2008.

HISTORY

POSIX.1-2001.

Prior to POSIX.1-2008, regoff_t was required to be capable of storing the largest value that can be stored in either an off_t type or a ssize_t type.

CAVEATS

re_nsub is only required to be initialized if REG_NOSUB wasn't specified, but all known implementations initialize it regardless.

Both regex_t and regmatch_t may (and do) have more members, in any order. Always reference them by name.

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(&regex, re, REG_NEWLINE))
        exit(EXIT_FAILURE);
    printf("String = \"%s\"\n", str);
    printf("Matches:\n");
    for (unsigned 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("#%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); }