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.

Perl Weekly’s review

When it comes to Perl regex, we get plenty of varieties. Thanks for the cool naration and DIY tool. Keep it up great work.

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['']);
}

6 lines of code

Output from script

Input:  'abccccd'
Output: 'cccc'

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

Input:  'abcdd'
Output: ''

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

Input:  'iiwwiiwwiiwwii'
Output: ''