Multisite Setup: Redirect non-subdomain content to root domain

I had a site for several years with lots of posts that is not a multi-site. We no longer want traffic to the root domain, but would like to redirect traffic to old posts to our landing page where users select a city first.

What htaccess rewrite conditions will redirect all http://www.domain.com/ and domain.com/ traffic to http://www.domain.com?

For example if someone goes to http://www.domain.com/year/post-title they should be redirected to http://www.domain.com, but if should not redirect subdomain.domain.com/year/post-title

Thanks!