I've tried to explain this topic in a couple of classes that I teach, but I can do no better than Andrei Zmievski's post on SlideShare. This guy's post is so excellent - it is THE best education source on Regular Expressions that I think I've ever seen.
That said, sometimes as a consultant, I don't have access to the Internet and thus can't get to this site, or I just need a quick and dirty lookup and don't have time to wade through a presentation like this.
In that situation I refer to my pocket regex guide. The one I have currently has bent pages, permanently opens to the Java section, and the pages are all stained from my dirty paws, but its been worth its weight in gold.
Labels: java, regex