def _extract_url_and_title(self, text, start):
        """Extracts the url from the tail of a link."""
        # text[start] equals the opening parenthesis
        idx = self._whitespace.match(text, start + 1).end()
        if idx == len(text):
            return None, None
        end_idx = idx
        has_anglebrackets = text[idx] == "<"
        if has_anglebrackets:
            end_idx = self._find_balanced(text, end_idx+1, "<", ">")
        end_idx = self._find_balanced(text, end_idx, "(", ")")
        match = self._inline_link_title.search(text, idx, end_idx)
        if not match:
            return None, None
        url = text[idx:match.start()]
        if has_anglebrackets:
            url = self._strip_anglebrackets.sub(r'\1', url)
        return url, end_idx
Extracts the url from the tail of a link.