Task — Group position

You are given a $string of lowercase letters. Write a script to find the position of all groups in the given string. Three or more consecutive letters form a group. Return '' if none found.

Examples

Example 1
Input: $str = 'abccccd'
Output: 'cccc'

Example 2
Input: $str = 'aaabcddddeefff'
Output: 'aaa', 'dddd', 'fff'

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

Analysis

The trick in this challenge is to find a regular expression that will return both the character that forms the group and the length of the group. A little experimentation yielded this:

$string =~ m|(.)(\1\1+)|g

This says 'any character, followed by the same character, followed by the same character one or more times'. The group is then returned in $1$2. You need the two bracketed patterns because the second one is defined using the result of the first.

I believe that doing it that way may mean that the regular expression engine has to be invoked repeatedly at run time, but I can't imagine the effect is substantial.

Try it 

#!/usr/bin/perl

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

use v5.26;    # The Weekly Challenge - 2025-04-21
use utf8;     # Week 318 - task 1 - Group position
use warnings; # Peter Campbell Smith
binmode STDOUT, ':utf8';
use Encode;

group_position('abccccd');
group_position('aaabcddddeefff');
group_position('abcdd');
group_position('xxxooxoxoxoxooo');
group_position('iiwwiiwwiiwwii');

sub group_position {
    
    my ($string, $result);
    
    $string = shift;
    $result .= qq['$1$2', ] while $string =~ m|(.)(\1\1+)|g;
    
    say qq[\nInput:  '$string'];
    say qq[Output: ] . ($result ? substr($result, 0, -2) : q['']);
}

Output

Input:  'abccccd'
Output: 'cccc'

Input:  'aaabcddddeefff'
Output: 'aaa', 'dddd', 'fff'

Input:  'abcdd'
Output: ''

Input:  'xxxooxoxoxoxooo'
Output: 'xxx', 'ooo'

Input:  'iiwwiiwwiiwwii'
Output: ''