MongoDB php $in e $regex

Ecco il mio breve snippet:

$user_query = preg_replace("/[[:blank:]]+/"," ", $user_query);
$arr_query = explode(' ', $user_query);

if (count($arr_query) > 1) {
    $tmp = array();

    foreach ($arr_query as $q) {
        $tmp[] = new MongoRegex( "/". $q ."/" );
    }

    $who['keywords'] = array('$in' => $tmp);

} else {
    $who['keywords'] = new MongoRegex( "/". $user_query ."/" );
}

$db->collection->find( $who );