public function css($string)
    {
        if (!$string) {
            return $string;
        }

        $string = $this->toUtf8($string);
        if ($string === '' || ctype_digit($string)) {
            return $string;
        }

        $result = preg_replace_callback('/[^a-z0-9]/iSu', $this->cssMatcher, $string);
        return $this->fromUtf8($result);
    }