Not helpful at all for learning, but I've found RegExper[1] is fantastic for visualizing existing expressions, particularly ones you haven't written but need to understand or edit.

[1] http://www.regexper.com/

This is also a great tool to play around with / construct regexes: http://regex101.com/

for JavaScript devs: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guid...

also http://overapi.com/regex