Membership - URL Group Regular Expression - Block all sub pages except..

I'm using Membership on a site that uses bbPress and I would like to block all the forums from non-paying users except a "Public" forum section I have set up.

Want to block all forums under:
http://domain.com/forums/forum/

example: http://domain.com/forums/forum/some-forum-category/private-forum

But I want to allow access to any forums under the "Public" forum category:
http://domain.com/forums/forum/public/

example: http://domain.com/forums/forum/public/another-forum

I was going to try and create both a positive and a negative rule for URL groups but it looks like the plugin doesn't allow this.
Can you tell me what I would put in the URL group to accomplish this?

Thank you.