Virtual Subdomain Mapping To Querystring Variable

Hi all,

Sorry if this is an idiot question but I'm an idiot.

I have a site which uses third-party data which I'm trying to localise using virtual subdomains mapped to one variable in the querystring, for example:

london.mysite.com would map to mysite.com/index.php?region=45

I'm using a php-generated rewrite map. Problem I'm having is, I'd like the rewrite rule to apply to any page on the site, but preserve the rest of the querystring, so where I currently have:

mysite.com/index.php?wldsite=2917&mygender=2&gender=1&minage=18&maxage=50&region=45&x=120&y=20

I'd like it to map to

london.mysite.com/index.php?wldsite=2917&mygender=2&gender=1&minage=18&maxage=50&x=120&y=20

and

mysite.com/tellafriend.php?wldsite=2917&mygender=2&gender=1&minage=18&maxage=50&region=45&x=120&y=20

to

london.mysite.com/tellafriend.php?wldsite=2917&mygender=2&gender=1&minage=18&maxage=50&x=120&y=20

Is this possible just using mod_rewrite, or am I wasting my time?