Hi there,
Sorry we didn't spot your question in the Forums - usually we keep on top of these things. We use the Regex flavour implemented in .NET, which is documented here: http://msdn.microsoft.com/en-us/library/az24scfc(v=VS.100).aspx For more details, hit the blue "?" in the alert configuration for Long-running query alerts, which should help to get you started. Finally, the exact Regex you're looking for is:
WAITFOR
Literally just this word will exclude all queries that include "WAITFOR" in their query text.
Hope that helps,
Daniel