RegExr

Here's a great little site that lets you test regular expressions. The splash page says:

Welcome to RegExr 0.3b, an intuitive tool for learning, writing, and testing Regular Expressions. Key features include:

• real time results: shows results as you type
• code hinting: roll over your expression to see info on specific elements
• detailed results: roll over a match to see details & view group info below
• built in regex guide: double click entries to insert them into your expression
• online & desktop: regexr.com or download the desktop version for Mac, Windows, or Linux
• save your expressions: My Saved expressions are saved locally
• search Community expressions and add your own
• create Share Links to send your expressions to co-workers or link to them on Twitter or your blog [ex. http://RegExr.com?2rjl6]

Built by gskinner.com with Flex 3 [adobe.com/go/flex] and Spelling Plus Library for text highlighting [gskinner.com/products/spl].