How to redirect non-www to www without hardcoding using .htaccess?

RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteCond %{HTTP_HOST} ^www\.(.+)$ [NC]
RewriteRule ^(.*)$ http://%1/$1 [R=301,L]

...causes a perfect, non-hardcoding 301 redirect from "www to non-www", what would the exact opposite look like?

EDIT:

According to Prix' post I've changed the .htaccess file to the following:

Options +FollowSymLinks
<IfModule mod_rewrite.c>
  RewriteEngine On
  RewriteBase /
  RewriteCond %{HTTPS} !=on
  RewriteCond %{HTTP_HOST} !^www\.(.+)$ [NC]
  RewriteRule ^(.*)$ http://www.%1/$1 [R=301,L]
</IfModule>

As already mentioned, this redirects to http://www./ unfortunately. Who can help?

Prix almost had it. When you negate the RewriteCond (with !) it doesn't capture so %1 is empty. Two possible solutions:

Dummy RewriteCond:

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

%{HTTP_HOST} in RewriteRule:

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

In summation, a clean, tested version of the code:

This works (for me) to redirect www to non-www

RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteCond %{HTTP_HOST} ^www\.(.+)$ [NC]
RewriteRule ^(.*)$ http://%1/$1 [R=301,L]

Either one of these work (for me) to redirect non-www to www

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

or

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

RewriteCond %{HTTP_HOST} !^www\.(.+)$ [NC]
RewriteCond %{HTTP_HOST} ^(.+)\.(.+)$ [NC]
RewriteRule ^(.*)$ http://www\.%2/$1 [R=301,L]

the ! means if it does not start with "www..." then send it to www.%1 which is the (.+)