Retrieves the text matching the
parentheses. 0 is the full text of the match, 1 is the first paren
set, 2 the second, and so on.
match_num is a valid sub pattern but it didn’t match anything
(e.g. sub pattern 1, matching “b” against “(a)?b”) then an empty
string is returned.
If the match was obtained using the DFA algorithm, that is using
g_regex_match_all() or g_regex_match_all_full(), the retrieved
string is not that of a set of parentheses but that of a matched
substring. Substrings are matched in reverse order of length, so
0 is the longest match.
The string is fetched from the string passed to the match function, so you cannot call this function after freeing the string.