Javascript/Regex for finding just the root domain name without sub domains

Don't use regex, use the .split() method and work from there.

var s = domain.split('.');

If your use case is fairly narrow you could then check the TLDs as needed, and then return the last 2 or 3 segments as appropriate:

return s.slice(-2).join('.');

It'll make your eyes bleed less than any regex solution.

You can't do this with a regular expression because you don't know how many blocks are in the suffix.

For example google.com has a suffix of com. To get from subdomain.google.com to google.com you'd have to take the last two blocks - one for the suffix and one for google.

If you apply this logic to subdomain.google.co.uk though you would end up with co.uk.

You will actually need to look up the suffix from a list like http://publicsuffix.org/