Regular Expression Pocket Reference

A good reference for Regular Expressions with sections detailing various implementations, (.NET, Python, Perl, etc.). This will help when I need a quick reminder of a regex sequence. It is not tutorial though. Some of the descriptions of technicalities are difficult to understand, because there is no explanation in places, where a little explanation would go a long way. For example the book frequently points out that a particular express will not match a certain string, but does not make it clear why it does not match. I spent about two hours trying to understand why "(?>[ab]*)\w\w" will match aabbcc, but not aabbaa. (It's because the first group matches the whole string, and won't back up to allow the last two word characters to match the end of the string.) Actually I probably learned my lesson better than if this had been spelled out, but I don't think that was the design of the text.

Xxx2020