Adding www. before HTTP::host / regexp matching

Hmm... Well, leaving aside the fact that you shouldn't have it as a CNAME (Because as a domain you'll have an SOA record plus maybe an MX record, and you shouldn't have any other records if you have a CNAME record). Best create it as an A record instead... Anyway...

IIUC what you're trying to do is if the host is NOT www.domain.com then do the redirect? Sorry, I'm not sure what regex syntax you're using there, but it doesn't look right... Would

 
  if {!(string tolower [HTTP::host]] equals "www.domain.com") } {  
 

work just as well? If you just to to match when the host isn't www.domain.com? Using a regex for a simple match seems a bit overkill to me.

H