Redirect only domain.com to www.domain.com and retain sub-domains

[web2008]

I want to redirect only domain.com to www.domain.com and retain sub-domains.

Example:
http://example.com -> http://www.example.com
http://m.example.com -> http://m.example.com

[edited by: phranque at 9:29 am (utc) on Jan 20, 2014]
[edit reason] exemplified domain [/edit]

[lucy24]

What have you got so far?

:: insert "Why We Make You Do It Yourself" boilerplate ::