G'day BernieC,

I would have used index for this (rather than the previously suggested substr).

With index, you are doing exactly what you show in your spec: is the character at a certain position the one you want. With substr, you are extracting a character at a certain position, and then performing a comparison operation.

The amount of coding for both is the same:

$ perl -E '
    my @x = qw{ABC DEF BXB};
    my ($i, $c) = qw{1 B};

    for (@x) {
        say "$_: ", $i == index($_, $c, $i) ? "YES" : "NO";
        say "$_: ", $c eq substr($_, $i, 1) ? "YES" : "NO";
    }
'
ABC: YES
ABC: YES
DEF: NO
DEF: NO
BXB: NO
BXB: NO
[download]

A Benchmark indicated that index was roughly twice as fast as substr. I used much the same code as I showed above:

#!/usr/bin/env perl

use strict;
use warnings;

use Benchmark 'cmpthese';

my @x = qw{ABC DEF BXB};
my ($i, $c) = qw{1 B};

cmpthese 0 => {
    index => sub { $i == index($_, $c, $i) ? 1 : 0 for @x },
    substr => sub { $c eq substr($_, $i, 1) ? 1 : 0 for @x },
};
[download]

I ran this three times. All results were very close. The middle was:

            Rate substr  index
substr 3981142/s     --   -46%
index  7383385/s    85%     --
[download]