[.htaccess] .htaccess - how to force "www." in a generic way?

This will change domain.com to www.domain.com:

# Force the "www."
RewriteCond %{HTTP_HOST} !^www\.domain\.com$ [NC]
RewriteRule ^(.*)$ http://www.domain.com/$1 [R=301,L]

How do I replace the "domain" part so that this works on any domain?

If you want to redirect all non-www requests to your site to the www version, all you need to do is add the following code to your .htaccess file:

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

RewriteEngine On

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

This redirects example.com to www.example.com excluding subdomains.

This is an older question, and there are many different ways to do this. The most complete answer, IMHO, is found here: https://gist.github.com/vielhuber/f2c6bdd1ed9024023fe4 . (Pasting and formatting the code here didn't work for me)

This won't work with subdomains.

domain.com correctly gets redirected to www.domain.com

but

images.domain.com gets redirected to www.images.domain.com

Instead of checking if the subdomain is "not www", check if there are two dots:

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

this worked like magic for me

RewriteCond %{HTTP_HOST} ^sitename.com [NC] RewriteRule ^(.*)$ https://www.sitename.com/$1 [L,R=301,NC]

This will do it:

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

The following should prefix 'www' to any request that doesn't have one, and redirect the edited request to the new URI.

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