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If the last regular expression search found a match, this function returns the position of the start of the matching text or of a subexpression of it.
If COUNT is zero, then the value is the position of the start of the entire match. Otherwise, COUNT specifies a subexpression in the regular expression, and the value of the function is the starting position of the match for that subexpression.
The value is ‘nil’ for a subexpression inside a ‘\|’ alternative that wasn’t used or a repetition that repeated zero times.