Editing and Testing Regular Expressions with RegExr
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.
RegExr is built with Flex 3, and uses ActionScript 3's built in RegExp engine. It isn't as powerful as a product like RegExBuddy, but it has the advantage of being online and free. You are able to save your patterns locally, and to share patterns on a searchable community database of regular expressions as well.