My favorite text editor of all time is ActiveState Komodo. I used to use the full/pro version of this but since I was too lazy to buy it so I kept redownloading the trial version every month. A while back ActiveState released a free version of the editor called Komodo Edit.
Komodo Edit has almost all the features I use. Almost. One feature that I missed pretty soon after changing was the RegExp editor. It is a pretty simple tool that allows you to type in a regular expression, some sample text and it would show you the groups etc. So, I decided I would just reimplement this in DHTML (no AJAX here).
The implementation is pretty simple. It uses input/propertychange events to track changes to the text areas and then applies the regular expression to the test text and displays the result in a tree. For the tree I used xTree2b which has been in beta for a few years now…
Updated: I added labels to the textfields and the result area.