[Plugin: Redirection] Help with regular expression

Hello,

I’m trying to do a catchall redirect so users landing on http://mydomain.com/ are redirected to another domain – yet still allow access to the WP admin areas:

http://mydomain.com/wp-login.php
http://mydomain.com/wp-admin/anything

Please can anyone help me write a regular expression to do this? My thinking is: redirect any URL that does NOT start with http://mydomain.com/wp. This is the closest I’ve got (with someone else’s help):

/(?!wp)(.*)

This redirects the front-end ok, allows me to view wp-login.php, login and then I get stuck in the redirect loop:

This webpage has a redirect loop
The webpage at http://mydomain.com/wp-login.php?redirect_to=http%3A%2F%2Fdomain.com%2Fwp-admin%2F&reauth=1 has resulted in too many redirects. Clearing your cookies for this site or allowing third-party cookies may fix the problem. If not, it is possibly a server configuration issue and not a problem with your computer.

Thanks in advance for any ideas.