public function validateVatNumberFormat(string $vatNumber) : bool
    {
        if ($vatNumber === '') {
            return false;
        }

        $vatNumber = strtoupper($vatNumber);
        $country = substr($vatNumber, 0, 2);
        $number = substr($vatNumber, 2);

        if (! isset($this->patterns[$country]) ) {
            return false;
        }

        $matches = preg_match('/^' . $this->patterns[$country] . '$/', $number) > 0;
        return $matches;
    }