public function normalize($string) {
        if ($string == '') return '';
        $parts = explode('%', $string);
        $ret = array_shift($parts);
        foreach ($parts as $part) {
            $length = strlen($part);
            if ($length < 2) {
                $ret .= '%25' . $part;
                continue;
            }
            $encoding = substr($part, 0, 2);
            $text     = substr($part, 2);
            if (!ctype_xdigit($encoding)) {
                $ret .= '%25' . $part;
                continue;
            }
            $int = hexdec($encoding);
            if (isset($this->preserve[$int])) {
                $ret .= chr($int) . $text;
                continue;
            }
            $encoding = strtoupper($encoding);
            $ret .= '%' . $encoding . $text;
        }
        return $ret;
    }