if zero, return the entire matched text; if positive integer, return the COUNTth matched parenthetical subexpression.
If the last such operation was done against a string with
string-match, then you should pass the same string as the argument
IN-STRING.
After a buffer search or match, you should omit IN-STRING or pass ‘nil’ for
it; but you should make sure that the current buffer when you call
match-string is the one in which you did the searching or matching.
The matched text or a portion of the matched text as a string depending on the value of COUNT.
The value is ‘nil’ if COUNT is out of range, or for a subexpression inside a ‘\|’ alternative that wasn’t used or a repetition that repeated zero times.