Apache - auto redirect the domain name to www url

How can I set that the URL of main domain to be redirected to www without subdomains.
For example: domain.hu --> redirect --> http://www.domain.hu
but the subdomains not: whatever.domain.hu --> no redirect --> http://www.whatever.domain.hu

I tried to modify this file: apache/parts/custom.conf.tpl

with the following content:

But after that the subdomains will be redirected too.

How can I configure this?

Maybe the custom.conf should be cut into two files (custom_domain.conf and custom_subdomain.conf) ?

It's not problem for me just I think that cool solution to set it in config template than setting it manually every time.