正则表达式 – 如何只保留Perl标量中的前五行？

#!/usr/bin/perl

use strict;
use warnings;

my $s = join '',map { "$_n" } 1 .. 9;

my ($first) = $s =~ /^((?:.*n){0,5})/;
my ($last) = $s =~ /((?:.*n){0,5})$/;


print "first:n${first}last:n$last";

一个更常见的解决方案是这样的：

#!/usr/bn/perl

use strict;
use warnings;

#fake a file for the example    
my $s = join '',map { "$_n" } 1 .. 9;    
open my $fh,"<",$s
    or die "could not open in memory file: $!";

my @first;
while (my $line = <$fh>) {
    push @first,$line;
    last if $. == 5;
}

#rewind the file just in case the file has fewer than 10 lines
seek $fh,0;

my @last;
while (my $line = <$fh>) {
    push @last,$line;
    #remove the earliest line if we have to many
    shift @last if @last == 6;
}

print "first:n",@first,"last:n",@last;