I got this case of an unefficient regexp handling when matching strings in a large file:

To look for ONE string takes 2 seconds
while looking for TWO strings takes 79 seconds. Here is the code:

use strict;
use Benchmark;

my $file = shift || 'no_file';

timethese(
    1,
    {
        'one_string'  => sub { one_string() },
        'two_string'  => sub { two_string() },
    }
);

sub one_string {
    my $filter = '00901808';
    my $re = qr/$filter/o;
    my @matched;
    open (my $FH, "<$file");
    while (my $rec = <$FH>) {
    if ( $rec =~ $re) {
            push @matched, $rec;
    }
    }
    close $FH;
}

sub two_string {
    my $filter = '00901808|87654321';
    my $re = qr/$filter/o;
    my @matched;
    open (my $FH, "<$file");
    while (my $rec = <$FH>) {
    if ( $rec =~ $re) {
            push @matched, $rec;
    }
    }
    close $FH;
}

__END__
[download]

# perl bench_regexp 100000lines.92MB.file 
Benchmark: timing 1 iterations of one_string, two_string...
one_string:  2 wallclock secs ( 1.68 usr +  0.42 sys =  2.10 CPU) @  0
+.48/s (n=1)
            (warning: too few iterations for a reliable count)
two_string: 77 wallclock secs (76.13 usr +  0.59 sys = 76.72 CPU) @  0
+.01/s (n=1)
            (warning: too few iterations for a reliable count)
[download]