private function matchUAAgainstKey($key, $userAgent = null){

        // Make the keys lowercase so we can match: isIphone(), isiPhone(), isiphone(), etc.
        $key = strtolower($key);
        $_rules = array_change_key_case($this->getRules());

        if(array_key_exists($key, $_rules)){
            if(empty($_rules[$key])){ return null; }
            return $this->match($_rules[$key], $userAgent);
        }

        return false;

    }