That's a very greedy regexp, with that .* in there and the /s modifier at the end. I bet that's your problem. Try creating a little mailbag and trying it on that--I bet it'll run, and you'll see how your regex fails.

(If you can't get it to work, tell us a little more about how those mail files are structured--I don't think it would be hard to make it work reliably, but I don't know a darn thing about Netscape mail files.)