This isn't precisely a perl-related question, but i didn't know where else to turn. I know some here are wizards with reg exp, so hopfully this will be a bit of a brain teaser for someone.
I have been wrangling with this for hours now, can't seem to get the reg exp right.
I need to put the following in .htaccess for a directory, so that domains that are IP aliased to this directory will not give a 404 error if the requested URL does not exist. basically I need to read the HTTP_HOST env variable, and strip everything except for domainname.com. In otherwords, if a user requests

http://anything.domainname.com

or
http://www.domainname.com/blahblah.html

the .htaccess rewrite will redirect them simply to

http://www.domainname.com

I have gotten it to work for

http://www.domainname.com/blahblah.html

and

http://anything.domainname.com

but I cannot for the life of me get it to work for
http://anything.domainname.com/blahblah.html

This is what I have so far:

RewriteEngine On
RewriteCond %{HTTP_HOST} ^(.*)\.([^.]+)\.com [NC]
RewriteRule ^(.*) http://www.%2/ [R=301,L]
[download]

Any help would be appreciated. TIA!