A more generalized way to handle such requirements is with a Finite-State Machine (FSM) approach. The algorithms consider, not only the current line of input, but the $state that the FSM is “in” at the time, where the current value of $state is determined by recent history of lines seen.

Sounds cool. I found a lot of interesting hits using Super Search:

• Re: Comparing two hashes-help: "I often find it useful to describe logic like this in terms of a finite-state machine (FSM)"
• Re: Spliting file + removing column: "For this, I use “finite-state machine (FSM)” logic"
• Re: ADSI groups users: "I naturally look at such problems with an eye toward so-called finite state machine logic"
• Re: IO::Socket client does not detect when server network connection dies: "Logic like this is sometimes well designed using Finite State-Machine (FSM) logic"
• Wisdom on how to build a "stressful simulation test" with Selenium & POE: "Each actor is basically an individual finite-state machine"
• Re: A way to avoid repeated conditional loops: "Call it “a flag variable” if you want to, but this is a classic place for a finite-state machine (FSM) algorithm"
• Re: Perl/Tk code structure: "A typical design for the shepherd process is a Finite-State Machine (FSM), or more likely, two FSMs"
• Re: how did blocking IO become such a problem?: "The entire life cycle of a request, and much of the outer request-handling heuristics, is most easily described using a finite-state machine (FSM) algorithm"
• Re: Clubbing array elements together:: "I prefer to solve such problems using a Finite-State Machine (FSM) algorithm"
• Re: File Find/Replace with the replacement coming from part of earlier matched string: "This is an absolutely classic case for a “finite-state machine (FSM)” algorithm"
• Re: How to check if successfully logged in?: "it must be a finite-state machine (FSM) design, because in the final analysis the host web-site is driving the bus ... Fact of the matter is, a production mechanize-script is often two FSMs"
• Re: Reading concurrently two files with different number of lines: "It might be useful for you to look at the concept of Finite-State Machine (FSM) algorithms as a source of ideas for generalized solutions to these problems"
• Re: RFC: Simulating Ruby's "yield" and "blocks" in Perl: "these can be used to implement finite-state machines (FSMs)"
• Re: Selecting HL7 Transactions: "this sort of thing is most-easily handled by finite-state machine (FSM) techniques"
• Re^2: Too much recursion: "For dealing with very complicated inputs, the notion of a Finite-State Machine (FSM) can be useful"
• Re: Sorting through a file with multiple tables and extracting data: "The general approach is that of a finite-state machine (FSM)"
• Re: Is this a simple, robust, and maintainable design?: "sounds like a Finite-state machine. If you haven't coded a finite state machine in Perl before, this article may be helpful."

All these glowing endorsements have got me excited, yet I couldn't find any sample code in any of these nodes. So I was wondering if you could post some of the excellent FSM code you've implemented over the years? It would really help me to better understand FSMs.