docs: def expect_re(self, regexp):
          """Test against the given regular expression and returns the match object.
  
          :param regexp: the expression to be tested.
          :returns: the match object.
          """
          prefix_len = self.match_prefix(
              self.prefix, self.next_line(require_prefix=False)
          )
          if prefix_len >= 0:
              match = self._expect_re(regexp, self.pos + prefix_len)
              self.match = match
              return match
          else:
              return None