Allow UPN or samAccountName for ActiveSync

I've seen several threads that appear to address this but do not. I need a variable assign that removes the domain from the logon name, I'm not successful with the TCL command.

I have a logon page/branch rule that changes based on the @ character. If @ is in the logon name, then the path uses ldap authentication and passes UPN, this auth works fine. But this prevents me from using the "split domain from username" option. Activesync configs send domain\username in the login process, so I need a variable assign to split the domain name and send only the username.