Alter username before Radius auth

I am trying to do somehting similar.

 

I want to change user@domain.com to domain\user

 

 

Looking for REGEX syntax assistance with my APM policy.

 

 

Something like:

 

session.user = expr { [regsub\(.*\)@\(.*\)/\2\\\1} [mcget {session.logon.last.username}] ""] }