This is highly technical, non-geeks run away, now.
Okay, no, just kidding, this is just a bookmark entry since this is so cool.
Regular expressions are part of the Zen™© of the Programming. They are patterns which are matched against a string and check whether there is a match or not, take parts of the string or similar.
Also, regexps are highly geeky because they are an absolutely unreadable mix of all kinds of punctiation. Simple regexes are easy to write and easy to understand, however there are some whose complexity requires more understanding than one can spare for an average human lifetime.
Here come the RegEx debuggers. Funny, I almost have used none so far, maybe I like to break my brain on them. Anyway, there was one which was full of "meta matches" (?:) and recursive groups (eek), and I was looking for the easy way and googled "regex debugger". Found a lot of interesting but not that useful one, but then…
Nice. Syntax diagram of the regexp.