Regexp in C - match group

Question

I\'ve been struggling with regular expressions in C (just /usr/include/regex.h).

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I have (let\'s say) hundreds of regexps and one of them can match

Answer 1

I assume your regex_match is some combination of regcomp and regexec. To enable grouping, you need to call regcomp with the REG_EXTENDED flag, but without the REG_NOSUB flag (in the third argument).

regex_t compiled;
regcomp(&compiled, "(match1)|(match2)|(match3)", REG_EXTENDED);

Then allocate space for the groups. The number of groups is stored in compiled.re_nsub. Pass this number to regexec:

size_t ngroups = compiled.re_nsub + 1;
regmatch_t *groups = malloc(ngroups * sizeof(regmatch_t));
regexec(&compiled, str, ngroups, groups, 0);

Now, the first invalid group is the one with a -1 value in both its rm_so and rm_eo fields:

size_t nmatched;
for (nmatched = 0; nmatched < ngroups; nmatched++)
    if (groups[nmatched].rm_so == (size_t)(-1))
        break;

nmatched is the number of parenthesized subexpressions (groups) matched. Add your own error checking.