I don't believe there's a way to do this currently. According to this SO answer, full regex is not supported in the exclude patterns, only wildcards (*). Additionally, my own testing revels that paths that begin or end with a slash are not supported.
I've opened a topic on the Sublime Issue tracker to request better path support. Issue 332. I recommend adding a comment there to increase its visibility to the Sublime devs.
I kinda doubt we'll ever see full regex support, due to performance concerns, but I hope we'll eventually get something better than what we have now.
In the meantime, one workaround is to use folder_include_patterns instead. Given your particular example, you could do something like the following. It's is not very elegant, and would need to be updated manually when you add new top-level folders, but it'd work.