A Javascript regex to match a regex was posted to Stackoverflow (formerly computer programming) by Mike Samuel:
/\/((?![*+?])(?:[^\r\n\[/\\]|\\.|\[(?:[^\r\n\]\\]|\\.)*\])+)\/((?:g(?:im?|m)?|i(?:gm?|m)?|m(?:gi?|i)?)?)/
Along with a nifty explanation and caveats about its use.
Other candidates?
Thinking this could be very useful for mining regexes out of discussion groups, etc.