A simple state machine will do. You are looking for either the start tag or for the end tag, so process each line in one of those contexts. 

#!/usr/bin/perl

use strict;
use warnings;

my $START = "1";
my $END   = "2";

my $looking_for_end = 0;
my @buffer;

while (<DATA>) {
   if ($looking_for_end) {
      # Start tag -> Abort and restart
      if ($_ eq "$START\n") {
         print(@buffer);
         @buffer = ($_);
      }

      # End tag -> Remove newlines
      elsif (substr($_, -(length($END)+1)) eq "$END\n") {
         print(grep { $_ ne "\n" } @buffer);
         print($_);
         @buffer = ();
         $looking_for_end = 0;
      }

      # Newline -> Add to buffer (in case we abort)
      elsif ($_ eq "\n") {
         push(@buffer, $_);
      }

      # Unexpected -> Abort
      else {
         print(@buffer);
         print($_);
         @buffer = ();
         $looking_for_end = 0;
      }

   } else {

      # Start tag -> Start buffering and looking for end tag.
      if ($_ eq "$START\n") {
         @buffer = ($_);
         $looking_for_end = 1;
      }
      else {
         print($_);
      }
   }
}

__DATA__


1

b

1




_____ 2

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In reply to Re: Implementing a parsing rule in a huge data file by ikegami
in thread Implementing a parsing rule in a huge data file by thezip

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