.htaccess to require WWW for domain, but allow sub

Question

I'm trying to figure it out how to set up an .htaccess set of rules that would force the presence of the "www" in front of the domain if it is not initially specified, but at the same time, it will not have any effect if the an subdomain is pressent; all this without hard coding any domain name so that the script is portable around different servers and configurations.

EDIT:

I'm sorry I was not able to get this explained right in the fist place. So what I need is as follows:

http://example.com -> redirects to http://www.example.com

http://www.example.com -> does not redirect

http://subdomain.example.com -> does not redirect

Answer 1

This mod_rewrite rule should do it for second level domains:

RewriteCond %{HTTP_HOST} ^[^.]+\.[^.]+$
RewriteCond %{HTTPS}s ^on(s)|
RewriteRule ^ http%1://www.%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

Answer 2

RewriteCond %{HTTP_HOST} ^example\.org
RewriteRule ^ http://www.example.org%{REQUEST_URI} [L,R=301]

This will not redirect subdomains like mail.example.org