RegExr: Free Online RegEx Testing Tool
Grant Skinner have released a Utility Tool -- RegExr, built with Flex 3, and uses ActionScript 3's built in RegExp engine.
RegExr is an online tool for editing and testing Regular Expressions (RegExp / RegEx). It provides a simple interface to enter RegEx expressions, and visualize matches in real-time editable source text. It also provides a handy RegExp snippet sidebar with descriptions and usage examples to make it easier to learn Regular Expressions through trial and error.
So far this has only taken a day of development, and the main app is only 150 lines of code. Flex 3 makes this kind of app so darn simple to put together.
RegExr is available online and is free. Grant have promised to release a Free Desktop version built with AIR in a day or two. So, the desktop version can be used on Mac, Windows and Linux (in the near future). The online version requires Flash Player 9.