From the webpage:
RegExr is an online tool to learn, build, & test Regular Expressions (RegEx / RegExp).
- Results update in real-time as you type.
- Roll over a match or expression for details.
- Save & share expressions with others.
- Explore the Library for help & examples.
- Undo & Redo with Ctrl-Z / Y.
- Search for & rate Community patterns.
For fast text processing, very little can touch regexes and Unix command line utilities.
I first saw this at Nathan Yau’s Learn regular expressions with RegExr.