Retrieves the text matching the
match_num'th capturing 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 match_all or 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.
number of the sub expression
The matched substring, or null if an error occurred. You have to free the string yourself