Try this if your files are too big for your memory: 
#!/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;

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This creates a temporary copy of the data file so you have to make sure that enough space is on disk. Or preferably check the return values of the print statements for errors.

In reply to Re: replace fist and last occurrences of N by jethro
in thread replace fist and last occurrences of N by ini2005

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