protected function reserveTag($content, $tag)
    {
        if (stripos($content, '<' . $tag) !== false) {

            if (in_array(strtolower($tag), array('img'))) {
                $pattern = '~(<' . preg_quote($tag, '~') . '[^>]+\>)~i';
            } else {
                $pattern = '~(<' . preg_quote($tag, '~') . '\\b[^>]*?>[\\s\\S]*?<\\/' . preg_quote($tag, '~') . '>)~i';
            }

            return preg_replace_callback($pattern, array($this, 'placeHolder'), $content);
        }

        return $content;
    }