APM Variable assign

Hi DC Experts,

I am using the following expression in my policy (Variable Assign)

session.logon.last.username = expr {[string range [mcget {session.server.landinguri}] [mcget {session.custom.pos_username}] [mcget {session.custom.pos_at_sign}]] }

But when I am trying save, it is allowing me to do.

Is there any mistakes in my expression. Please advise me.