Problem Statement

Create a function which takes two string parameters: 'stones' and 'jewels' and returns an integer. Both strings can contain any number of upper or lower case letters. However, in the case of 'jewels', all letters must be distinct. The function should count (and return) how many 'stones' are 'jewels' or, in other words, how many letters in 'stones' are also letters in 'jewels'.

Note that: So, for example, if passed "aAAbbbb" for 'stones' and "aA" for 'jewels', the function should return 3. This task was inspired by this problem.

Let's start with the solution:

sub count_jewels {
    my( $j, $s ) = @_;
    my($c,%S);

    $S{$_}++     for split //, $s;
    $c += $S{$_} for split //, $j;
    return "$c\n";
}

print count_jewels 'aA' , 'aAAbbbb';
print count_jewels 'z'  , 'ZZ';


#!/usr/bin/perl

use strict; # https://rosettacode.org/wiki/Jewels_and_Stones#Perl
use warnings;

sub count_jewels { scalar( () =  $_[0] =~ /[ $_[1] ]/gx ) } # stones, jewels

print "$_ = ", count_jewels( split ), "\n" for split /\n/, <<END;
aAAbbbb aA
aAAbbbb abc
ZZ z
END