How to replace the hostname from a FQDN in HTTP:HOST

I probably did not clear explain what I need.

For example:

we have some domains...

www.test1.com

www.test2.com

Now I want to get only the domains to create a new one like:

In case of www.test1.com

mobile.test1.com

In case of www.test2.com

mobile.test2.com

etc...

This should be more dynamically then with a "switch" method:

I think should be something like...

set dom [HTTP:Host]

extract only the domain in this case test1.com build the new FQDN.

I hope I did clearly describe what I mean.

Many thanx

andy