There are already plenty good answers. Adding another answer for the fun of it. This answer uses the CPAN Module File::ReadBackwards.

It reads the file backwards and prints out the results every time a block is complete (Title line found). Memory usage should therefore be low. Exit if the expected "Title" line is not found for $MAX_LENGTH chars. Otherwise large input files without "Title" lines could still cause massive memory usage.

Caveat: the length is only updated every time a "line" has been read. If there is a huge input line (e.g. several gigabytes without newline), this programm will still crash and burn :). 

use strict;
use warnings;
use File::ReadBackwards;

my $MAX_LENGTH = 1_000_000;
my $infile     = shift @ARGV;

die "'$infile' is not a file!" unless (-f $infile);

my $bw = File::ReadBackwards->new( $infile )
         or die "can't read '$infile' $!";

my (@block, $length, $line);

while( defined( $line = $bw->readline ) ) {
    unshift @block, $line;
    $length += length $line;

    if ($line =~ m/^Title\b/) {
        print @block;
        @block  = ();
        $length = 0;
    }

    if ($length >= $MAX_LENGTH) {
        die "$length chars read, without finding a 'Title' line\n".
            "Input file '$infile' probably has the wrong file format."
+;
    }
}

if (@block) {
    print STDERR scalar(@block) ." lines not printed out yet!";
}

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In reply to Re: How to reorder a text file by rminner
in thread How to reorder a text file by hennesse

Title:
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