public static function cleanRut(string $rut, bool $forceUppercase = true)
    {
        // Filter the RUT string and return only numbers and verification digit.
        $filtered = preg_filter('/(?!\d|k)./i', '', $rut) ?? $rut;

        // If the filtered RUT is not empty and over the 6 characters, we're good.
        if (!empty($filtered))
            return $forceUppercase ? strtoupper($filtered) : strtolower($filtered);
    }