Since most of use don't get to use Regular Expressions on a regular basis, tools to help us write them can make a huge difference to our sanity. Especially, if your trying to write anything complex that you can't find the solution to with a simple Google search. Here are a few simple tools that may help writing those fun and challenging regex.
Before I start listing out the tools here is a quick warning. Regular Expressions tend to be slightly different depending on the library or language your using. Make sure the tool you use to help you write them works for the language your programming in or you might run into some headaches. I learned this lesson the hard way.
Firefox Regular Expression Tester
Want to write regex? There is a Firefox extension for that. The Regular Expression Tester does what it's name implies. It lets you test regular expressions on some text. It's simple, fast, and right in the favorite browser of many developers.
On my regex primer post, Robert DeBoer pointed out RegExr Desktop. It's an Adobe AIR application for writing, storing, and sharing regular expressions. Because it's written in for AIR it'll run on Windows, Mac, and Linux.
RegExhibit is a Mac only program that's a little bit older. But, it's simple, slick, and it just works.
Since so many developers have turned to using Macs (I even know some who write Windows programs on Macs) it should be no surprise that there are dashboard widgets for writing regular expressions. Two of them are the Regex Widget and Regex Survival Toolkit.
These are hardly an exhaustive list and you'll notice a lack of Windows programs on this list. Since I don't use Windows for development it's hard to point out any good programs for it. But, there are a bunch of free ones.