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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:

^ # Start at the beginning of the string
( # Capture
(?: # Complex expression:
(?!www) # make sure we're not at the beginning of www
. # accept any character
)* # any number of times
) # End capture
\.domain\.com # and ending at .domain.com