RegExr is a web tool for testing those widely used (yet often confusing) regular expressions. Regular expressions (regex) are search patterns that help you find strings of text in e-mail messages, text files, etc.
Things like Usenet search tools, Google Code search, and Open Office support regex, and regex versatility far surpasses simple wildcard searches like *.exe. For instance, a regular expression like .*@([^.]+\.)*example.com$ will find all email addresses from example.com (and its subdomains).
To use RegExr, type your expression in the top box, and then type/insert text in the box below to see if it matches your expression. You can use the helpful sidebar to learn about and insert different regex strings to build your overall pattern.
If you want a more feature-rich program, are running Windows, and have $39.95 to spare, try
RegExBuddy. The RegExr developer plans to release free AIR versions of his product for Mac OS X and Windows.