index to the subexpression to return. 0 corresponds to the entire regular expression.
returns the content of the n'th subexpression of the last successful use of the =~ regex matching operator. Returns an empty string if the n'th subexpression was not matched (e.g. due to alternation). Throws an error if the last use of =~ was a failed match, or if fewer than n subexpressions are present in the original regexp.