Regular expressions are an incredibly powerful tool, but can be rather tricky to get exactly right. This is a website that I wrote so I could quickly and easily test regular expressions during development.

Pick which programming language you are using:

Go Haskell Java JavaScript .Net Perl PHP Python Ruby Tcl XRegExp

Share codes are an easy way to share regexes. If you have a share code, enter it here:
A badly formed regular expression can run forever, making it very useful for denial of services attacks. Some of the engines/hosts are susceptible to these. Check the current engine status to see if they are running.
The original version was Java-based and did not require a JavaScript-enabled browser. No need to use it if the Java one above works for you, but a lot of links go directly to it:

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One last thing

Some people, when confronted with a problem, think “I know, I’ll use regular expressions.” Now they have two problems.
  -- Jamie Zawinski in comp.lang.emacs (or somewhere else).
I define UNIX as “30 definitions of regular expressions living under one roof”.
  -- Donald Knuth in Digital Typography, ch. 33, p. 649 (1999) via re2.