I found http://www.regular-expressions.info/ http://www.regular-expressions.info/ to be an invaluable resource when learning or understanding regular expression

I found http://www.regular-expressions.info/ http://www.regular-expressions.info/ to be an invaluable resource when learning or understanding regular expressions. Basically every page that explains a feature in the reference also explains what's going on within the engine, how it works, when it backtracks, etc. Those things are sometimes hard to see in applications that just give you the matches from a text (as rubular.com seems to do).