If you're using the expect command to wait for one of a series of inputs within your expect program, information will be checked in the program's internal buffers inthe following order:
a) Any string that matches expect_before will be found
b) Matches to each of the possible patterns in expect in turn
c) Matches to expect_after.

Note that you can't be certain how much information will have been written to the internal buffer up to any point, so you need to exercise care when specifying the possible sequences.

Once expect_before expect_after or expect matches as string, the section up to that string is transferred to the expect_out(buffer) variable that you can reference in your continuing Tcl, and that section is removed from the internal bugger. The following expect command will start looking from where the previous successful match was found.

Note that the -notransfer option to expect will prevent the matched section from being removed from the internal buffer.

The expect_background command allows you to specifiy an expect command that runs, in effect, in an asyncronous background loop. You can use it to receive inputs from processes that may be responding from time to time while you're doing other processing in the foreground, without having to go into a wait loop until one of those background processes responds.
(written 2007-10-27, updated 2007-10-30)

 T211 - Tcl/Tk - What is Expect? Why use it?
  [3572] Adding Expect on top of Tcl - what is it and where can I get a training course to learn about it? - (2012-01-08)
  [3286] Should we cover expect and/or Tk on our public Tcl courses? - (2011-05-11)
  [3009] Expect in Perl - a short explanation and a practical example - (2010-10-22)
  [2489] Parallel Pinging, using Python Threads or Expect spawn lists - (2009-11-02)
  [2474] Using Tcl and Expect to automate repetitive jobs - (2009-10-24)
  [1602] Automating processes through Expect - (2008-04-05)
  [1531] Expecting a item from a list of possibles - (2008-02-04)
  [1469] Curley brackets v double quotes - Tcl, Tk, Expect - (2007-12-12)
  [1409] What is Expect? - (2007-10-26)
  [1174] Installing Tcl and Expect on Solaris 10 - a checklist - (2007-05-02)
  [1173] Cheat Sheet / Check list for Expect maintainers - (2007-05-02)
  [435] Expect for Windows - (2005-09-04)
  [286] Automating regular manual procedures - (2005-04-21)

T242 - Tcl/Tk - More on Expect
  [3448] Checking all the systems on a subnet, using Expect and Tk - (2011-09-18)
  [2475] Quick easy and dangerous - automated logins via Tcl / Expect - (2009-10-24)
  [1475] Tcl/Tk - updating your display while tasks are running - (2007-12-16)

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Better technology makes for longer commutes?
What we teach - expained for the non-technical
The little gestures that can really count
Sparse and Greedy matching - Tcl 8.4
Buffering of inputs to expect, and match order
Tcl / regsub - changing a string and using interesting bits
Wireless hotel tips - FTP and Skype connections failing
Reading from another process in Tcl (pipes and sockets)
Away or home - which do I prefer?