I really don't know from memory how to do that regex code. However, I think to get any subdomain from that format you could use something like:
$url = "http://dg.booo.com/index.jsp?action=viewRegion%AEionId=3&cid=1554";
$explode = explode(".", $url);
$explode2 = explode("//", $explode[0]);
$subdomain = $explode2[1];
Herk