A side-note to start, I have been developing software for over 12 years. I started in C, then came C++, Java (6 years), C#, PHP, the whole shabang. Overall, the best IDE I’ve ever used is Intellij Idea. It is simply brilliant, or used to be some 5 years ago. It introduced Refactoring before anyone else, cleaned up your code for you, removed excessive spaces, just made everything tidier.
Although I consider Eclipse to be a good choice, especially coupled with FDT from PowerFlasher, Eclipse is bloated. Intellij Idea used less than 30 mb of RAM way back then, and I consider an IDE should never use any more than 50 mb of RAM, even for the most complex projects.
FlashDevelop is a brilliant alternative. It has gotten significantly better this last year, with true AS3 support, incredible auto-complete, auto-import and now even setter/getter generation built-in. I has a RegEx panel too, I think that one is a plug-in tho.
Basically Mika & Philippe are doing an amazing job. Plus it’s FREE.