private function _parseAttrType(int &$offset): string
    {
        $idx = $offset;
        // dotted OID
        $type = $this->_regexMatch('/^(?:oid\.)?([0-9]+(?:\.[0-9]+)*)/i', $idx);
        if (null === $type) {
            // name
            $type = $this->_regexMatch('/^[a-z][a-z0-9\-]*/i', $idx);
            if (null === $type) {
                throw new \UnexpectedValueException("Invalid attribute type.");
            }
        }
        $offset = $idx;
        return $type;
    }