#!/usr/bin/perl -w

use strict;
use warnings;

my $filename = shift;

open my $fh,'<',$filename or die "No input file\n";
open my $ofh, '>', "$filename.new" or die "Can't open output file\n";

my $previousline= undef;

while (<$fh>) {
    chomp;
    s/(?<=[^n])n|n(?=[^n])/^/ig; #borrowed from jwkrahn
    if (defined $previousline) {
	($previousline=~/[^n]$/i) and (s/^n/^/i);  
	(/^[^n]/i) and ($previousline=~s/n$/^/i);  
	print $ofh $previousline,"\n";
    }
    $previousline= $_;
}

print $ofh $previousline,"\n" if (defined $previousline);

close $fh; close $ofh;

rename "$filename.new",$filename;