There is a famous quote about regular expressions, to which I don’t really agree but I have to admit there is not much love for regexps around me.

Some people, when confronted with a problem, think “I know, I’ll use regular expressions.” Now they have two problems.
Jamie Zawinski

I came across this little tool : Regexper allowing to easily understand a regular expression by providing its state machine diagram :

Regexper example

I think it could be useful to put it in your bookmarks with Rejex to use it when you work with regular expressions.