Retrieves the number of matched substrings (including substring 0, that is the whole matched text), so 1 is returned if the pattern has no substrings in it and 0 is returned if the match failed.
If the last match was obtained using the DFA algorithm, that is
g_regex_match_all() or g_regex_match_all_full(), the retrieved
count is not that of the number of capturing parentheses but that of
the number of matched substrings.