Using conditional statements inside ‘expect’

Have to recomment the Exploring Expect book for all expect programmers — invaluable.

I’ve rewritten your code: (untested)

proc login {user pass} {
    expect "login:"
    send "$user\r"
    expect "password:"
    send "$pass\r"
}

set username spongebob 
set passwords {squarepants rhombuspants}
set index 0

spawn telnet 192.168.40.100
login $username [lindex $passwords $index]
expect {
    "login incorrect" {
        send_user "failed with $username:[lindex $passwords $index]\n"
        incr index
        if {$index == [llength $passwords]} {
            error "ran out of possible passwords"
        }
        login $username [lindex $passwords $index]
        exp_continue
    }
    "prompt>" 
}
send_user "success!\n"
# ...

exp_continue loops back to the beginning of the expect block — it’s like a “redo” statement.

Note that send_user ends with \n not \r

You don’t have to escape the > character in your prompt: it’s not special for Tcl.