public static function html_tag_callback($matches)
    {
        // It might be an ending of a comment, ignore (#6464)
        if (substr($matches[3], -2) == '--') {
            $matches[0] = '';
            return implode('', $matches);
        }

        $tagname = $matches[2];
        $tagname = preg_replace(array(
            '/:.*$/',                // Microsoft's Smart Tags <st1:xxxx>
            '/[^a-z0-9_\[\]\!?-]/i', // forbidden characters
        ), '', $tagname);

        // fix invalid closing tags - remove any attributes (#1489446)
        if ($matches[1] == '</') {
            $matches[3] = '';
        }

        return $matches[1] . $tagname . $matches[3];
    }