private function _parseAttrHexValue(int &$offset): string
    {
        $idx = $offset;
        $hexstr = $this->_regexMatch('/^(?:[0-9a-f]{2})+/i', $idx);
        if (null === $hexstr) {
            throw new \UnexpectedValueException("Invalid hexstring.");
        }
        $data = hex2bin($hexstr);
        $offset = $idx;
        return $data;
    }