I thought about trying but it's hard to dip into the regex data structure (if there is one) and also display it in some sort of meaningful way. Other than that

I thought about trying but it's hard to dip into the regex data structure (if there is one) and also display it in some sort of meaningful way. Other than that, Regetron is kind of just enough of a tool to get you through the book and have no friction. I think for more advanced regex exploration there's http://kodos.sourceforge.net/ http://kodos.sourceforge.net/ and I believe there's a few javascript ones in the browser and even a few very advanced ones in Java.