The Regex Coach is a graphical application for Linux and Windows which can be used to experiment with (Perl-compatible) regular expressions interactively. It has the following features:

• It shows whether a regular expression matches a particular target string.
• It can also show which parts of the target string correspond to captured register groups or to arbitrary parts of the regular expression.
• It can "walk" through the target string one match at a time.
• It can simulate Perl's split and s/// (substitution) operators.
• It tries to describe the regular expression in plain English.
• It can show a graphical representation of the regular expression's parse tree.
• It can single-step through the matching process as performed by the regex engine.
• Everything happens in "real time", i.e. as soon as you make a change somewhere in the application all other parts are instantly updated.

Homepage: http://www.weitz.de/regex-coach/

Incredibly useful application for those who just can't wrap their minds around this stuff. (actually, one was to wonder whether anybody can. I bet they're all fakin it!)

Only thing it needs is to be able to nest regexes (eg: okay, so that's what it'll look like when it's split, but what happens if I s/foo/wiki/g the results of the split?)