You have to keep a buffer of previously read lines large enough to retain the nth previous line you are wanting to print. How to handle the situation where there aren't n lines before your found line is an exercise left to the reader.

$ perl -Mstrict -Mwarnings -E '
open my $inFH, q{<}, \ <<EOD or die $!;
line 1
line 2
line 3
line 4
line 5
line 6
line 7
line 8
line 9
EOD

my $lineBeforeMatch = 5;
my @prevLines;

my $lookFor = qr{line 7};

while ( <$inFH> )
{
    push @prevLines, $_;
    shift @prevLines if $#prevLines > $lineBeforeMatch;
    print $prevLines[ 0 ] if m{$lookFor};
}'
line 2
$

[download]

I hope this is helpful.

Cheers,

JohnGG

In reply to Re: Printing line before matching expression by johngg
in thread Printing line before matching expression by rm

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