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:


But I want to allow access to any forums under the "Public" forum category:


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.