Task — Greatest English letter

You are given a string, $str, made up of only alphabetic characters [a..zA..Z]. Write a script to return the greatest English letter in the given string. A letter is greatest if it occurs as lower and upper case. Also letter ‘b’ is greater than ‘a’ as ‘b’ appears after ‘a’ in the English alphabet.

Examples

Example 1
Input: $str = 'PeRlwEeKLy'
Output: L

There are two letters E and L that appears as lower 
and upper. The letter L appears after E, so the L is the 
greatest English letter.

Example 2
Input: $str = 'ChaLlenge'
Output: L

Example 3
Input: $str = 'The'
Output: ''

Analysis

I can think of several ways of meeting this challenge, but I think this one is probably the most efficient.

• Sort the characters of $str in reverse (eg cbaCBA)
• Loop over the lower case letters ($c) in the sorted string
• Look for a match of $c.*$C where $C == uc($c)

Because of the reverse sort the first match will be the match which is the last alphabetically, and thus the greatest English letter in the string.

Try it 

#!/usr/bin/perl

# Blog: http://ccgi.campbellsmiths.force9.co.uk/challenge

use v5.26;    # The Weekly Challenge - 2024-04-08
use utf8;     # Week 264 - task 1 - Greatest English letter
use warnings; # Peter Campbell Smith
binmode STDOUT, ':utf8';

greatest_english_letter('PeRlwEeKLy');
greatest_english_letter('zebras live in London Zoo');
greatest_english_letter('Antelopes like chocolate');
greatest_english_letter('all lower case gives a null result');
greatest_english_letter('the lower case can come fiRst');
greatest_english_letter('maybe harder - yAaBbcCdDY');

sub greatest_english_letter {
    
    my ($str, $C, $c, $m);
    
    $str = shift;
    say qq[\nInput:  \$str = '$str'];
    
    # reverse sort characters of $str
    $str = join('', sort { $b cmp $a } split('', $str));
    
    # loop over lc chars in $str
    while ($str =~ m|([a-z])|g) {
        $C = uc($1);
        
        # test for uc same char
        next unless $str =~ m|$1.*$C|;
        say qq[Output: '$C'];
        return;
    }
    say qq[Output: ''];
}

Output

	
Input:  $str = 'PeRlwEeKLy'
Output: 'L'

Input:  $str = 'zebras live in London Zoo'
Output: 'Z'

Input:  $str = 'Antelopes like chocolate'
Output: 'A'

Input:  $str = 'all lower case gives a null result'
Output: ''

Input:  $str = 'the lower case can come fiRst'
Output: 'R'

Input:  $str = 'maybe harder - yAaBbcCdDY'
Output: 'Y'