code: def matching_tokens(self, text, start=0):
          
          for token_class, regexp in self._tokens:
              match = regexp.match(text, pos=start)
              if match:
                  yield token_class, match
docs: Retrieve all token definitions matching the beginning of a text.
  
          Args:
              text (str): the text to test
              start (int): the position where matches should be searched in the
                  string (see re.match(rx, txt, pos))
  
          Yields:
              (token_class, re.Match): all token class whose regexp matches the
                  text, and the related re.Match object.