Return an args string for the repr.

def args_str(self):
        """
        Return an args string for the repr.
        """
        matched = [str(m) for m in self._matchers[:self._position]]
        unmatched = [str(m) for m in self._matchers[self._position:]]
        return 'matched=[{}], unmatched=[{}]'.format(
            ', '.join(matched), ', '.join(unmatched))