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10.3.3 Matching a Compiled POSIX Regular Expression

Once you have compiled a regular expression, as described in POSIX Regexp Compilation, you can match it against strings using regexec. A match anywhere inside the string counts as success, unless the regular expression contains anchor characters (‘^’ or ‘$’).

— Function: int regexec (const regex_t *restrict compiled, const char *restrict string, size_t nmatch, regmatch_t matchptr[restrict], int eflags)

This function tries to match the compiled regular expression *compiled against string.

regexec returns 0 if the regular expression matches; otherwise, it returns a nonzero value. See the table below for what nonzero values mean. You can use regerror to produce an error message string describing the reason for a nonzero value; see Regexp Cleanup.

The argument eflags is a word of bit flags that enable various options.

If you want to get information about what part of string actually matched the regular expression or its subexpressions, use the arguments matchptr and nmatch. Otherwise, pass 0 for nmatch, and NULL for matchptr. See Regexp Subexpressions.

You must match the regular expression with the same set of current locales that were in effect when you compiled the regular expression.

The function regexec accepts the following flags in the eflags argument:

REG_NOTBOL
Do not regard the beginning of the specified string as the beginning of a line; more generally, don't make any assumptions about what text might precede it.
REG_NOTEOL
Do not regard the end of the specified string as the end of a line; more generally, don't make any assumptions about what text might follow it.

Here are the possible nonzero values that regexec can return:

REG_NOMATCH
The pattern didn't match the string. This isn't really an error.
REG_ESPACE
regexec ran out of memory.