Re: obtaining the three leftmost characters of a string

[joshua]

my $phone = "555-4587";
$phone =~ /(\d\d\d)/;
my $first_three = $1;

Joshua

[jeffa]

You can combine the last two lines:

my ($first_three) = $phone =~ /(\d{3})/;

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)

[RhetTbull]

Egads! A regex where substr would suffice! Shame, shame!

Update: OK, this was a good-natured jest but it seems at least a few people take issue with my above remark. So, anyway, here is some data to backup my comment. First of all, the poster asked specifically how to find the leftmost three characters of a string. His example showed a number (presumably a phone number) but the title of the post did not limit the problem domain to phone numbers. The poster asked for "the three leftmost characters" and the answer given was "the first three digits" which are not the same thing -- hence my good natured jest. This is a perfect problem for substr. Sure, the regex works fine, but it's overkill. It's like pulling out a chainsaw when all you need is a butter knife. substr is also more than twice as fast.

regex: 37.940104 seconds
substr: 14.430599 seconds

The benchmark code that produced the above results is:

#!/usr/bin/perl

use strict;
use warnings;
use Time::HiRes qw/ gettimeofday tv_interval /;

#load up the numbers
open my $fh, "< nums.out" or die;
my @numbers = <$fh>;
my $first_three;
my ($start, $stop, $run_time);

my $count = 100;

$start = [gettimeofday()];
for (0..$count)
{
    foreach my $phone (@numbers) 
    {
        ($first_three) = $phone =~ /(\d{3})/;
    }
}
$stop = [gettimeofday()];
$run_time = tv_interval($start,$stop);
print "regex: $run_time seconds\n";

$start = [gettimeofday()];
for (0..$count)
{
    foreach my $phone (@numbers) 
    {
        $first_three = substr $phone, 0, 3;
    }
}
$stop = [gettimeofday()];
$run_time = tv_interval($start,$stop);
print "substr: $run_time seconds\n";

I didn't use Benchmark because it didn't like it when I tried to pre-load the numbers (it's a file of 10,000 strings that match the poster's example). I wanted to pre-load to avoid file I/O messing up the benchmark.