Redirect all non www to www except one subdomain with htaccess

I'd like to redirect all URLs not containing www to www

I used this:

RewriteCond %{HTTP_HOST} !^www.example.com$
RewriteRule ^(.*) http://www.example.com$1 [QSA,L,R=301]

Now I need one subdomain not to be redirect like this, or I get an error.

So I'd like to redirect any URL that does not begin with sub or with www to www.example.com

How can I do this ? thanks