private function scanEscapedUnicodeSequence(): string
    {
        static $hexChars = "0123456789ABCDEFabcdef";
        static $length = 4;

        $sequence = $this->scanWhile($hexChars, $length);

        if (\strlen($sequence) < $length) {
            throw new ParseException($this->getExceptionMessage());
        }

        return $sequence;
    }