private function highlight($words, $str) {

        $highlight = '<span class="highlight">$0</span>';
        $pattern = '#(?!<.*?)(%s)(?![^<>]*?>)#i';
        foreach ($words as $word) {
            $word = preg_quote($word);
            $regex = sprintf($pattern, $word);
            $str = preg_replace($regex, $highlight, $str);
        }
        return $str;
    }