Blocking a period at a specific location using RegEx

Blocking a period at the end of a whitelisted URL?

I have a RegEx which is partly working to help verify a white listed list of domains, with '|' as delimiter, is working

              string regEx3 = @"https?://(" + whitelist + ")(.*)\\?(goto|returnurl)=https?://(" + whitelist + ")";

But I need to BLOCK a domain which uses my white listed domain as a sub-domain.


sso.mydomain.com is okay, but the following is not
sso.mydomain.com.badURL.com


I need to make the RegEx fail when it discovers a period after the white listed domain.

How?

Thanks