Is Regex Hero completely compatible with the .NET flavor of regular expressions?

Yes. Regex Hero runs directly off of the Regex class library inside Silverlight. I will say that Silverlight is missing the RegexOptions.Compiled option that's normally in .NET. And this feature can affect performance. However, in terms of compatibility and functionality, Silverlight's Regex implementation is otherwise identical to the Regex inside the complete .NET framework.

How do I get multiline matching to work?

There are a couple things that might trip you up here. First, the ^ and $ anchors will normally assert the beginning and end of the entire string. You'll have to check the "multiline" option in Regex Hero for ^ and $ to match the beginning an end of every line. Second, there's the "CrLf marks a line ending" setting. If you check this then a carriage return/line feed combination will end every line. Otherwise, only line feeds are used. This is unchecked by default but you may want to change it under special circumstances.

What if I don't have Silverlight?

Have no fear, download it here. It's under 5 MB and extremely lightweight. It doesn't add any annoying system tray icons or any kind of agent to your computer. It's a pure and simple browser plug-in and it's definitely worth the download.