How can I assign a variable using $expect_out in TCL/EXPECT?

Question

If I want to match DEF_23 using the following regexp:

expect {
    -re \"DEF_\\[0-9]*\"
    set result $expect_out(1,string)
}

why

Answer 1

It seems that the above explication is not precise! Check this example:

$ cat test.exp
#!/usr/bin/expect

set timeout 5
log_user 0

spawn bash

send "ls -1 db*\r"
expect {
  -re "^db.*$" {
    set bkpfile $expect_out(0,string)
  }
}

send_user "The filename is: $bkpfile\n"

close
$ ls -1 db*
dbupgrade.log
$ ./test.exp
can't read "bkpfile": no such variable
    while executing
"send_user "The filename is: $bkpfile\n""
    (file "./test.exp" line 15)
$

The test result is the same when $expect_out(1,string) or $expect_out(buffer)is used. Am I missing something or this is the expected behavior?