public static function isLatinLetter($letter)
    {
        // Combining marks are a subset of non-spacing-mark.
        if (preg_match('/\p{L}/u', $letter) !== 1 && preg_match('/\p{Mn}/u', $letter) !== 1) {
            return false;
        }

        return (preg_match('/\p{Latin}/u', $letter) === 1)
        || (preg_match('/\pM+/u', $letter) === 1);
    }