Nice string

You are given a string made up of lower and upper case English letters only. Write a script to return the longest substring of the given string which is nice.

A string is nice if, for every letter of the alphabet that the string contains, it appears both in uppercase and lowercase.

Examples

Example 1
Input: $str = 'YaaAho'
Output: 'aaA'

Example 2
Input: $str = 'cC'
Output: 'cC'

Example 3
Input: $str = 'A'
Output: ''
No nice string found.

Analysis

I chose to approach this challenge in what could be called the systematic - or boring - way.

I loop over the possible starting characters, and then over the possible lengths of substrings which are larger than the nicest substring found so far. I then check each of these for niceness, and if one is nice, I save it as the new nicest.

I chose to pull out the test for a substring being nice into a separate subroutine is_nice() mainly to make the code clearer to read.

The post does a great job of highlighting ambiguities in specifications. It identifies potential edge cases (a9a09a) and makes reasonable assumptions while also pointing out how easily such details can be overlooked or underspecified.

Try it 

#!/usr/bin/perl

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

use v5.26;    # The Weekly Challenge - 2025-07-07
use utf8;     # Week 329 - task 2 - Nice string
use warnings; # Peter Campbell Smith
binmode STDOUT, ':utf8';
use Encode;

nice_string('YaaAho');
nice_string('cC');
nice_string('A');
nice_string('ABCDEedcbaZABCDEedcbaa');

# longer example
my (@chars, $string);
@chars = (('A' .. 'I'),('a' .. 'i'));
$string .= $chars[int(rand(18))] for 0 .. 50;
nice_string($string);

sub nice_string {
    
    my ($string, $c, $best, $longest, $s, $t, $try);
    
    # initialise
    $string = shift;
    $c = length($string);
    $best = '';
    $longest = 0;
    
    # loop over starting points
    for $s (0 .. $c - 2) {
        
        # loop up to remaining length
        for $t (0 .. $c - $s) {
            next unless $t >= $longest;
            $try = substr($string, $s, $t);
            
            # nicest so far?
            if (is_nice($try)) { 
                $best = $try;
                $longest = $t;
            }
        }
    }
    
    say qq[\nInput:  '$string'];
    say qq[Output: '$best'];
}

sub is_nice {
    
    my ($string, @chars, $c, $C);
    
    # initialise
    $string = shift;
    @chars = split('', $string);
    
    # check for nicety
    for $c (@chars) {
        $C = $c gt 'Z' ? uc($c) : lc($c);
        return 0 unless $string =~ m|$C|;
    }
    return 1;
}

23 lines of code

Output from script

Input:  'YaaAho'
Output: 'aaA'

Input:  'cC'
Output: 'cC'

Input:  'A'
Output: ''

Input:  'ABCDEedcbaZABCDEedcbaa'
Output: 'ABCDEedcbaa'

Input:  'IDbaDfiIDCDFHeEdHeFbGEGgbeBHdCAfBHbFfHIgCfiifAAI'
Output: 'bGEGgbeB'