Regular expressions in expect

Question

I just started learning expect scripting.I have been trying to extract the following from my output:

core.4046140998.01.10.133211

with an expect script using the following command.can somebody please tell me where I am going wrong?I want to get to store this entire string i.e.( core.4046140998.01.10.133211*) in a variable and perform some action with it.

expect -re {^(core)\.*} {puts $expect_out(0,string)}

Do i have to import any packages in expect to make this work?

Answer 1

Since this is expect, "core" may appear at the beginning of a line, but not at the beginning of the input string. To demonstrate:

$ expect
expect1.1> spawn sh
spawn sh
8043
expect1.2> send "echo core.1234\r"
expect1.3> exp_internal 1
expect1.4> expect -re {^core.*}
Gate keeper glob pattern for '^core.*' is 'core*'. Activating booster.

expect: does "" (spawn_id exp6) match regular expression "^core.*"? Gate "core*"? gate=no
sh-4.3$ echo core.1234
core.1234
sh-4.3$ 
expect: does "sh-4.3$ echo core.1234\r\ncore.1234\r\nsh-4.3$ " (spawn_id exp6) match regular expression "^core.*"? Gate "core*"? gate=yes re=no
expect: timed out
expect1.5> expect -re {(?n)^core.*}
Gate keeper glob pattern for '(?n)^core.*' is 'core*'. Activating booster.

expect: does "sh-4.3$ echo core.1234\r\ncore.1234\r\nsh-4.3$ " (spawn_id exp6) match regular expression "(?n)^core.*"? Gate "core*"? gate=yes re=yes
expect: set expect_out(0,string) "core.1234\r"
expect: set expect_out(spawn_id) "exp6"
expect: set expect_out(buffer) "sh-4.3$ echo core.1234\r\ncore.1234\r"
expect1.6> puts ">>>$expect_out(0,string)<<<"
<<<core.1234

Things to note:

• expecting -re {^core.*} did not match. We see the "timed out" message

• note what we're attempting to match:

  expect: does "sh-4.3$ echo core.1234\r\ncore.1234\r\nsh-4.3$ " (spawn_id exp6) match regular expression "^core.*"? Gate "core*"? gate=yes re=no
  # ............^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  

  It starts with the command I sent, so using a "normal" anchor won't work

• the next thing I expect is -re {(?n)^core.*}. This does match.

  • the (?n) is a little-used Tcl regex instruction that tells the regex engine we want "newline-sensitive" matching.
  • newline-sensitive matching means that . will not match a newline and (more relevant here) that ^ can match immediately after a newline within a multi-line string (similarly for $)
• note that the output of my puts command looks odd. That's due to the carriage return at the end of $expect_out(0,string). Be aware of that, and use string trim as required

The take-away lessons here are:

• extracting the output of commands can be hard in expect because the prompt and the sent command can get in the way.
• use expect debugging to see why your patterns aren't matching.