The site written by @wchung, RegexOne seems to be a perfect answer to the original question. Check that out first ;)
The tool txt2re is, as commenters have rightly mentioned, very ugly. RegExr (ht @runrunraygun) is a much prettier tool for editing regular expressions, although has completely different functionality and still no tutorial.
The main reason it might be useful for learning regexes is the ability to see the effect each edit is having on the matching.
Original answer:
I always thought txt2re was a very clever tool. You just type an example of some text you want to match, and then it lets you pick bits out to match against. I'd consider it to be a fairly useful tool for learning regular expressions.