$html = '[downloads_box title="big boobs"]http://www.domain.com/files/files.avi.html[/downloads_box]';
$regex = '/\[(\[?)(downloads_box)\b([^\]\/]*(?:\/(?!\])[^\]\/]*)*?)(?:(\/)\]|\](?:([^\[]*+(?:\[(?!\/\2\])[^\[]*+)*+)\[\/\2\])?)(\]?)/s';
$new = preg_replace($regex,' replacement',$html);
echo $html . 'will be replaced with' . $new;