Apache rewrite based on subdomain

You should have a look at the URL Rewriting Guide from the apache documentation.

The following is untested, but it should to the trick:

RewriteCond %{HTTP_HOST} ^([^.]+)\.blah\.domain\.com$
RewriteRule ^/(.*)$           http://blah.domain.com/%1/$1 [L,R] 

This only works if the subdomain contains no dots. Otherwise, you'd have to alter the Regexp in RewriteCond to match any character which should still work due to the anchoring, but this certainly feels safer.

Try this:

RewriteCond %{HTTP_HOST} (.+)\.blah\.domain\.com
RewriteRule ^(.+)$ /%1/$1 [L]

@pilif (see comment): Okay, that's true. I just copied a .htaccess that I use on one of my projects. Guess it has a slightly different approach :)