It will return "me+you" as a result. So with "PrimaryEmail" you will have to do it like this:
I was looking for something using the built in expression functions( I wasn't clear), but I may have to give yours a try. I just don't speak RegEx and hate to work blind.
I was trying
based on the Find and Substring functions
Find(str, strToFind, startingPosition)
but it's returning -1 as if not finding the @ in the email address