Ran across a "watch what your expressions do" tool while looking for something else. Haven't had the time (or motivation - as I've been using EditPadPro which does most of what this does) to install and test.
But this is freeware and may help someone struggling with RE - and I didn't find it back when I really could have used something like this...
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PS: there's also "regex buddy" that helps you build REs - and shows what the RE you're building does - but it's not freeware...