code: def _extract_url_and_title(self, text, start):
          
          # 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
docs: Extracts the url from the tail of a link.