Tcl Expect: variable send response to complex authentification prompt

Question

This is an excerpt of the relevant parts from a TCL script known to be working with a Telnet server.

#!/bin/sh
# \
exec tclsh "$0" ${1+"$@"}
package require Expect
#USER
set user xyz
set secret 12345678
set smr AB1CDE-11
set host foo.bar
#TELNET START
spawn telnet $host
expect "\[Login :\]"
send "$user\r";
expect "=>"
#RMS
send "c $smr gate\r";
expect "$smr >"
send "PW $secret\r"
expect "\[>\]"

The login procedure has changed a couple of days ago. Response with send "PW $secret\r" is not possible anymore. After the $smr > response, any command could be send to initiate login, e.g. send "login\r". The telnet responds with Login [517]:. The numbers inside the [] are corresponding to the position in $secret. A full $secrect response does not succeed anymore. The server expects a response which contains all three characters plus three additional characters.

For some reasons, I want the position of the characters from $secret to be at a random position among three random characters (possible chars in RegEx terms: [A-Z0-9.!@#$%^&*()_]). The length of $secret is variable. So I couldn't set a variable for each character to respond with them to a RegEx matchresult.

The right RegExp to capture the position of the characters would be LOGIN \[(\d)(\d)(\d)]:, right? I've got as far as the following (output is captured in a text file for statistical post-processing as I'm evaluating the randomness of the login procedure).

send "login\r"    
set input [open output_$user.txt a]
set output $expect_out(buffer)
puts $input $output
close $input
expect "LOGIN"
#Check LOGIN prompt
set file [read [open "output_$user.txt" r]]
if [
regexp -nocase {LOGIN \[(\d)(\d)(\d)]:} 
] then {
send "RESPONSE\r";
}

How I can I solve this issue with TCL? Thanks!

Ammendment:
After connecting to the Telnet Server with send "c $smr gate\r";

XYZ SMC via AB1CDE-11 >

is being replied, which is being expected by expect "$smr >". Until here, everything works fine.
To the following send "login\r", the Telnet server replies with two lines:

Login [517]:
XYZ SMC via AB1CDE-11 >

The matching RegEx would be Login \[(\d)(\d)(\d)\]:\n.*>. I've tried different expect RegEx syntaxes:
donal-fellows expect -re {Login \[(\d)(\d)(\d)\]:} Crash of TCL
expect -re -n {Login \[(\d)(\d)(\d)\]:\n.*>} Crash of TCL
expect {-re -n "Login \[(\d)(\d)(\d)\]:"} exit
Only something without regular expression (-re) like expect "$smr >" or expect "LOGIN" works, and TCL does'nt exit.
TCL appcompat (PATH of FOLDER has been anonymized):

<?xml version="1.0" encoding="UTF-16"?>
<DATABASE>
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</DATABASE>

Yes, I'm using Windows for this. Can't use Linux for this task due to this issue.
How shall I proceed?

Answer 1

Well, you can do some of it fairly easily by using a few more features of Expect. In particular, the expect command can wait for a regular expression to be matched and report what substrings match, and then use string index to pick the characters out of the secret.

expect -re {Login \[(\d)(\d)(\d)\]:}
# It's wise to copy these values from expect_out into other variables immediately
set digitA $expect_out(1,string)
set digitB $expect_out(2,string)
set digitC $expect_out(3,string)

# Is it zero-based or one-based indexing? Assuming the latter...
set charA [string index $secret [expr {$digitA - 1}]]
set charB [string index $secret [expr {$digitB - 1}]]
set charC [string index $secret [expr {$digitC - 1}]]

I'm not quite sure how you assemble those letters into the response — your description wasn't clear enough — but I'm sure you can figure that out now that you've got the raw materials.