I was puzzling out how to do some text processing in Tapestry and it came down to some regular expressions. Now, I use these all the time, but I'm not hyper literate in them ... I always need to test them out before I feel confident in them. I had been using an Eclipse plugin for this that went payware, so for I while I was firing up IRB (interactive Ruby).
Fortunately, during a quick thinking break, I decided to see if there was a new Eclipse tool for this ... and there is QuickREx.
It does a very good job ... it allows you to write your regular expression, provide it with sample text, and view matches and groups within matches very nicely. You can set options (such as multiline or caseless) and it even can run for all the major different RE implementations out there (JDK, ORO Perl, ORO Awk, and JRegex). It does live evaluation, which really helps when trying to "tweak" the expression, and has a bunch of other well thought out features, such as helping you to paste the final expression into Java code (escaping the backslashes, and such, for you).
The expression editor includes completion; hitting ctrl-space brings up a menu of different regular expression codes to insert, each with a snippet of documentation. Nice.
The plugin also includes a secondary view, the Regular Expression Library. This contains a library of regular expressions, each with example text to match against and a chunk of documentation.
In fact, there's a few more features that are hard to explain out of context, but I suspect will prove quite useful. This is another example of a finely crafted tool created not as a demonstration of someone's Eclipse plugin coding chops, but created to be used.
Easy install via the update manager http://www.bastian-bergerhoff.com/eclipse/features) and it's run flawlessly since.