private function _parsePairAfterSlash(int &$offset): string
    {
        $idx = $offset;
        if ($idx >= $this->_len) {
            throw new \UnexpectedValueException(
                "Unexpected end of escape sequence.");
        }
        $c = $this->_dn[$idx++];
        // special | \ | " | SPACE
        if (false !== strpos(self::SPECIAL_CHARS . '\\" ', $c)) {
            $val = $c;
        } else { // hexpair
            if ($idx >= $this->_len) {
                throw new \UnexpectedValueException("Unexpected end of hexpair.");
            }
            $val = @hex2bin($c . $this->_dn[$idx++]);
            if (false === $val) {
                throw new \UnexpectedValueException("Invalid hexpair.");
            }
        }
        $offset = $idx;
        return $val;
    }