Can awk patterns match multiple lines?

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Awk can easily detect multi-line combinations of patterns, but you need to create what is called a state machine in your code to recognize the sequence.

Consider this input:

how
second half #1
now
first half
second half #2
brown
second half #3
cow

As you have seen, it’s easy to recognize a single pattern. Now, we can write an awk program that recognizes second half only when it is directly preceded by a first half line. (With a more sophisticated state machine you could detect an arbitrary sequence of patterns.)

/second half/ {
  if(lastLine == "first half") {
    print
  }
}

{ lastLine = $0 }

If you run this you will see:

second half #2

Now, this example is absurdly simple and only barely a state machine. The interesting state lasts only for the duration of the if statement and the preceding state is implicit, depending on the value of lastLine. In a more canonical state machine you would keep an explicit state variable and transition from state-to-state depending on both the existing state and the current input. But you may not need that much control mechanism.

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