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.

With a bit of bashing I found a solution. Turns out that expect uses a TCL syntax that I'm not at all familiar with:

#!/usr/bin/expect
set pass(0) "squarepants"
set pass(1) "rhombuspants"
set pass(2) "trapezoidpants"
set count 0
set prompt "> "
spawn telnet 192.168.40.100
expect {
  "$prompt" {
    send_user "successfully logged in!\r"
  }
  "password:" {
    send "$pass($count)\r"
    exp_continue
  }
  "login incorrect" {
    incr count
    exp_continue
  }
  "username:" {
    send "spongebob\r"
    exp_continue
  }
}
send "command1\r"
expect "$prompt"
send "command2\r"
expect "$prompt"
send "exit\r"
expect eof
exit

Hopefully this will be useful to others.