问Expect脚本如何确定expect_out的内容

EN

#! /usr/bin/expect

set vpn    [ lindex $argv 0]
set group  [ lindex $argv 1]
set user   [ lindex $argv 2]
set secret [ lindex $argv 3]

log_user 2 
spawn nmcli con up $vpn --ask

expect {
        "GROUP:" { 
                send "$group\r"
                exp_continue
        }
        "Username:" { 
                send "$user\r"
                exp_continue
        }
        Password: {
                send_user "\nProvide RSA OTP:"
                expect_user -re ":(.*)\r"
                set otp $expect_out(0,string)  <--------- Error!!!
                set pass "$secret$otp"
                send "$pass\r"
                exp_continue
        }
        "Connection successfully activated" {
                send_user "Connected to VPN\r"
        }
        "Login failed" {
                send_user "Login failed\r"
                exp_continue
        }
        "already active" {
                send_user "Already connected to VPN\r"
        }
}
exit