protected static function getInvalidCharacters()
    {
        if (!self::$invalidCharString) {
            self::$invalidCharString = implode('', array_map('chr', array_merge(
                range(0, 32),
                array(34, 40, 41, 44, 47),
                array(58, 59, 60, 61, 62, 63, 64, 91, 92, 93, 123, 125, 127)
            )));
        }

        return self::$invalidCharString;
    }