Linux cli command regexec

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NAME 🖥️ regexec 🖥️

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",constchar*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 ([^…]) 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.

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;

John Doe;
John Foo; “; 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" “, str); printf(“Matches: “); 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: “, i); printf(“offset = %jd; length = %jd “, (intmax_t) off, (intmax_t) len); printf(“substring = "%.*s" “, len, s + pmatch[0].rm_so); s += pmatch[0].rm_eo; } exit(EXIT_SUCCESS); }