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Re: get n lines before or after a pattern

by Athanasius (Chancellor)
on Jul 26, 2012 at 03:32 UTC ( #983761=note: print w/replies, xml ) Need Help??

in reply to get n lines before or after a pattern

Here is another approach, using Tie::File:

#! perl use strict; use warnings; use Tie::File; my $file = 'test.txt'; tie my @lines, 'Tie::File', $file or die "Cannot tie file '$file': $!" +; for my $i (0 .. $#lines) { if ($lines[$i] =~ m{ \b jack \b }x) { for ($i - 2 .. $i) { print $lines[$_], "\n" unless $_ < 0; } for (my $found = 0; !$found && $i <= $#lines; ++$i) { if ($lines[$i] =~ m{ \b lastname \b }x) { print $lines[$i], "\n"; $found = 1; } } } } untie @lines;

What is nice about this approach is that, by treating the data file as an ordinary array, it is possible to meet more complicated requirements without the programming overhead of manually maintaining line buffers. So, this approach has the advantage of being scalable. Some notes on Tie::File:

  • It’s a core module: Tie::File
  • Written by Dominus
  • From the docs: “The file is not loaded into memory, so this will work even for gigantic files.”


Athanasius <°(((><contra mundum

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[ovedpo15]: another question about regex. if I have a string like this: "a/b/,c/d/e,5" I would like to remove the string after the last comma so the output will be "a/b/,c/d/e" how to do so?
[Corion]: ovedpo15: You could split on comma, and then join everything together, except the last part. Or you could match and make sure that the last part has no comma, like qr!,[^,]+$!, or you could use rindex.

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