protected function _insertPlaceHolder($matches)
    {
        $match = $matches[0];
        $envelope = ['', ''];
        if (isset($matches['url'])) {
            $match = $matches['url'];
            $envelope = [$matches['left'], $matches['right']];
        }
        if (isset($matches['url_bare'])) {
            $match = $matches['url_bare'];
        }
        $key = hash_hmac('sha1', $match, Security::getSalt());
        $this->_placeholders[$key] = [
            'content' => $match,
            'envelope' => $envelope
        ];

        return $key;
    }