r"""Guess encoding from xml header declaration.

        :param content: xml content
        :rtype: str or None

def guess_xml_encoding(self, content):
        r"""Guess encoding from xml header declaration.

        :param content: xml content
        :rtype: str or None
        """
        matchobj = self.__regex['xml_encoding'].match(content)
        return matchobj and matchobj.group(1).lower()