How to make sure domain.tld is always www.domain.tld

I know how to do this when a site doesn't have subdomains, but I have a domain now that also has subdomains and I want to redirect http://domain.tld to http://www.domain.tld and still allow for the various subdomains (http://sub.domain.tld) I have.

Any help would be appreciated, thanks!

 

try:

However, my sub-domain configuration is different to the usual, so this might not work onsome configurations

 

Here is mine :

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

Code (markup):