r"""Split tag namespace.

        :param tag: tag name
        :return: a pair of (namespace, tag)
        :rtype: tuple

def split_namespace(self, tag):
        r"""Split tag namespace.

        :param tag: tag name
        :return: a pair of (namespace, tag)
        :rtype: tuple
        """
        matchobj = self.__regex['xml_ns'].search(tag)
        return matchobj.groups() if matchobj else ('', tag)