By far the handiest Regex tool I’ve found for learning, writing, and testing Regular Expressions.
Key features include: (straight from the website)
- 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
http://gskinner.com/RegExr/