Force 'www.' through .htaccess except certain files

Question

Looking to redirect all traffic from http://example.com/ to http://www.example.com/

The catch is I have a few files that can't be redirected to http://www.example.com/

So example:

http://example.com/ to http://www.example.com/

http://example.com/test123 to http://www.example.com/test123

http://example.com/ip to htttp://example.com/ip