Task — Thousand separator

You are given a positive integer, $int. Write a script to add the thousands separator ',' and return it as a string.

Examples

Example 1
Input: $int = 123
Output: '123'

Example 2
Input: $int = 1234
Output: '1,234'

Example 3
Input: $int = 1000000
Output: '1,000,000'

Example 4
Input: $int = 1
Output: '1'

Example 5
Input: $int = 12345
Output: '12,345'

Analysis

Arthur Eddington

Two challenges here:

• Pattern matching that needs to start at the end of the string and work back, and
• Overlapping pattern matching.

Here's an easy way to overcome both of those:

• Reverse the string,
• Do a repeated replacemnt of 4 consecutive digits with the first 3 of them, a comma, and the last one.
• Reverse the result

Yes it's a bit inefficient, but the astronomer Sir Arthur Eddington is quoted as saying: 'I believe there are
15,747,724,136,275,002,577,605,653,961,181,555,468,044,
717,914,527,116,709,366,231,425,076,185,631,031,296
protons in the universe', which is probably the largest physically significant integer and it contains only 26 commas.

Try it 

#!/usr/bin/perl

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

use v5.26;    # The Weekly Challenge - 2026-01-05
use utf8;     # Week 355 - task 1 - Thousand separator
use warnings; # Peter Campbell Smith
binmode STDOUT, ':utf8';
use Encode;

thousand_separator(1);
thousand_separator(12);
thousand_separator(123);
thousand_separator(1234);
thousand_separator(123456);
thousand_separator(12345678901234567890);

sub thousand_separator {
    
    my ($number, $n, $reversed);
    
    # reverse the input
    $reversed = reverse($_[0]); 
    
    # repeatedly change 5555 to 555,5
    do { $n = $reversed =~ s|(\d{3})(\d)|$1,$2| } until $n == 0;
        
    say qq[\nInput:  $_[0]];
    say qq[Output: ] . reverse($reversed);
}

Output

Input:  1
Output: 1

Input:  12
Output: 12

Input:  123
Output: 123

Input:  1234
Output: 1,234

Input:  123456
Output: 123,456

Input:  12345678901234567890
Output: 12,345,678,901,234,567,890