regerror (3) - Linux Manuals

regerror: POSIX regex functions


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);


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:

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 ([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:
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.

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().


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.


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).
regcomp(), regexec() Thread safetyMT-Safe locale
regerror() Thread safetyMT-Safe env
regfree() Thread safetyMT-Safe


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


#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))

    printf("String = \"%s\"\n", str);

    for (int i = 0; ; i++) {
        if (regexec(&regex, s, ARRAY_SIZE(pmatch), pmatch, 0))

        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); }


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grep(1), regex(7)

The glibc manual section, Regular Expressions