Register a token class.

        Args:
            token_class (tdparser.Token): the token class to register
            regexp (optional str): the regexp for elements of that token.
                Defaults to the `regexp` attribute of the token class.

def register_token(self, token_class, regexp=None):
        """Register a token class.

        Args:
            token_class (tdparser.Token): the token class to register
            regexp (optional str): the regexp for elements of that token.
                Defaults to the `regexp` attribute of the token class.
        """
        if regexp is None:
            regexp = token_class.regexp

        self.tokens.register(token_class, regexp)