Note to reader:
Most likely you will find it little trouble to find a better (or better, briefer) regular expression tutorial within seconds using popular search engines.
If you do, you will most likely come to know regular expression rubrics well enough to spot at least three factual errors in this one, which is included merely for the convenience of having a cheat sheet on hand when typing up wikisheets (or other types of buffers) locally in Emacs.