[exim] Regex Help and Optimization

Heed a little regex help. I'm trying to grab the final domain part of a
string:

${sg{$sender_host_name}{\N^.*\.(.*\..*)$\N}{\$1}}

And this works - but it hits on *.domain.com and misses domain.com

How would I modify this to match domain.com too? Thanks in advance.