rewrite rule for subdomains only

I recently encountered a situation where I wanted a rewrite rule for isapi rewrite that would rewrite all subdomain urls, but leave the main www domain alone.

So for example:
test.domain.com
would be rewritten to
domain.com/?param=test

something.domain.com
would be rewritten to
domain.com/?param=something

But www.domain.com won't be rewritten at all.

After many desk-head based collisions I finally got it working: here's the rewrite rule you need to match subdomains as the hosts you are interested in, and not www:

RewriteCond Host: ^((?:(?!www).)*)\.domain\.com

Here's the explanation of how it works: