Retrieves the position in bytes of 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
end_pos are set to -1 and true is returned.
If the match was obtained using the DFA algorithm, that is using match_all or match_all_full, the retrieved position 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.
number of the sub expression
pointer to location where to store the start position, or null
pointer to location where to store the end position, or null
true if the position was fetched, false otherwise. If the position
cannot be fetched,