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The second paragraph in section 15.2 reads: "The Nth captured substring (where N > 0) is the string matched by the subexpression contained by the Nth left parenthesis in the regex. The zeroeth captured substring is the string that matches the entire regex. This means that the value of regex-group(0) is initially the same as the value of . (dot)." Is N < 0 an error?
The next sentence after your extract answers the question: The function returns the zero-length string if there is no captured substring with the relevant number.
I have added another item to the list of conditions under which a zero-length string is returned: * The number is negative.