Finally made it back with what I think would be a great idea...at least for the Linux users out here. A sed trainer/tester. SED uses regex expressions extensively, but most/all testers I have found show the regex as not matching, when (in fact) it does. I don't know if this is because it is a different version of regex, errors in the tester, or something else.
Ideally, what I would like to see is something where you point to a file to upload (or paste it in if it is a windows utility, which would be fine) then type in a sed statement. It would then show what the output would be without actually altering the file itself.
This idea came from a lot of Linux scripts I have been writing for work lately - scripts that must often alter existing configuration files. sed is a perfect tool for this, but it takes extensive time to research the proper regex to apply, only to find out it isn't exactly right. Moreover, once I DO find the right regex expression, making sure the replacement (most commonly), or other sed action, reacts properly is more trial and error.
For those unaware of the ever-powerful sed utility in Linux, this seminal work by Bruce Barnett provides a great detailed overview:http://www.grymoire.com/Unix/Sed.html
That said, reading it and doing it correctly (remembering all the rules, exceptions, switches, etc) are completely different, even when working with it on a regular basis. Hence a tool that would test, and perhaps even suggest solutions to find targeted text could be imminently helpful to linux scripters everywhere.