Task — Digital clock

You are given time in the format hh:mm with one missing digit. Write a script to find the highest digit between 0-9 that makes it a valid time.

Examples

Example 1
Input: $time = '?5:00'
Output: 1
Since 05:00 and 15:00 are valid time and no other digits can fit in the missing place.

Example 2
Input: $time = '?3:00'
Output: 2

Example 3
Input: $time = '1?:00'
Output: 9

Example 4
Input: $time = '2?:00'
Output: 3

Example 5
Input: $time = '12:?5'
Output: 5

Example 6
Input: $time =  '12:5?'
Output: 9

Analysis

There are six cases to consider:

1. The first digit is ? and the second is [0123] - the answer is 2
2. The first digit is ? and the second is [456789] - the answer is 1
3. The second digit is ? and the first is 1 - the answer is 9
4. The second digit is ? and the first is 2 - the answer is 3
5. The third digit is ? - the answer is 5
6. The fourth digit is ? - the answer is 9

This is a rather messy set of conditions. The easiest - and perhaps clearest - way of performing the task is just a set of if ... elsif clauses using the above logic, and that's what I submitted. I used split to put the characters into in array to make my conditions easy to read - for example $chars[0] eq '?' - but it could equally be done using regular expressions - for example
$string =~ m|^...\?| is true if the 4th character, ie third digit, is '?'.

Try it 

Script

#!/usr/bin/perl

# Peter Campbell Smith - 2022-12-06
# PWC 194 task 1

use v5.28;
use utf8;
use warnings;

my (@tests, $test, @chars, $result);

@tests = ('?5:00', '?3:00', '1?:00', '2?:00', '12:?5', '12:5?');

# loop over tests
for $test (@tests) {
    @chars = split(//, $test);
    
    if ($chars[0] eq '?') {        # ?1:30 or ?5:30
        $result = $chars[1] > 3 ? '1' : '2';
        
    } elsif ($chars[1] eq '?') {   # 1?:30 or 2?:30
        $result = $chars[0] eq '2' ? '3' : '9';
        
    } elsif ($chars[3] eq '?') {   # 11:?0
        $result = '5';
        
    } elsif ($chars[4] eq '?') {   # 11:3?
        $result = '9';
        
    } else {
        $result = 'invalid';
    }
    say qq[\nInput:  $test\nOutput: $result];
}

Output

Input:  ?5:00
Output: 1

Input:  ?3:00
Output: 2

Input:  1?:00
Output: 9

Input:  2?:00
Output: 3

Input:  12:?5
Output: 5

Input:  12:5?
Output: 9