Decode string

You are given an encoded string. Write a script to return the decoded string of the given encoded string.

The encoding rule is: K[encoded_string], where the encoded_string inside the square brackets is repeated exactly K > 0 times.

Examples

Example 1
Input: $str = '2[3[a]]'
Output: 'aaaaaa'
3[a]    => aaa
2[3[a]] => aaa aaa

Example 2
Input: $str = '10[a]'
Output: 'aaaaaaaaaa'

Example 3
Input: $str = 'a2[b]c3[d]e'
Output: 'abbcddde'

Example 4
Input: $str = '2[a2[b]c]'
Output: 'abbcabbc'

Example 5
Input: $str = '1[a]2[b3[c]]'
Output: 'abcccbccc'

Analysis

This is similar - though simpler - than week 387’s task 2.

I parse the input string left to right, keeping track of the current multiplier (initialised to 1) on a stack.

When I encounter a number followed by a '[' I push the old multiplier onto the stack and multiply the current multiplier by the number.

When I encounter one or more letters I output them for the current multiplier times.

When I find a ']' I pop the multiplier off the stack.

And that does it.

The contribution of Peter in Week 390 presents a creative stack-based technique which brings the conventional regex-replacement process for string decoding to a new level. The method uses a stack to keep track of the active multiplication factors and thus it demonstrates efficient state management.

This review may cover either or both challenges for this week.

Try it 

Your input:

#!/usr/bin/perl

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

use v5.26;    # The Weekly Challenge - 2026-09-07
use utf8;     # Week 390 - task 1 - Decode string
use warnings; # Peter Campbell Smith
binmode STDOUT, ':utf8';
use Encode;

decode_string('2[3[a]]');
decode_string('10[a]');
decode_string('a2[b]c3[d]e');
decode_string('2[a2[b]c]');
decode_string('1[a]2[b3[c]]');
decode_string('4[x3[y2[f]]]');
decode_string('co2[m]i2[t]2[e]');
decode_string('3[bang]');

sub decode_string {
    
    my ($f, $m, %c, @s, $o);
            
    # initialise
    $f = shift;
    say qq[\nInput:  '$f'];
    $m = 1;
    %c = ();

    # scan input
    while ($f) {
        
        # [ = increase multiplier
        if ($f =~ m|^(\d+)\[(.*)|) {
            push @s, $1;
            $m *= $1;
        
        # repeat letter $m times
        } elsif ($f =~ m|^([a-z]+)(.*)|) {
            $o .= $1 x $m;
        
        # ] = decrease multiplier
        } elsif ($f =~ m|^(\])(.*)|) {
            $m /= pop @s;
        
        } else {
            last;
        }
        $f = $2;
    }
    
    # report
    say qq[Output: '$o'];
}

18 lines of code

Output from script

Input:  '2[3[a]]'
Output: 'aaaaaa'

Input:  '10[a]'
Output: 'aaaaaaaaaa'

Input:  'a2[b]c3[d]e'
Output: 'abbcddde'

Input:  '2[a2[b]c]'
Output: 'aabbbbcc'

Input:  '1[a]2[b3[c]]'
Output: 'abbcccccc'

Input:  '4[x3[y2[f]]]'
Output: 'xxxxyyyyyyyyyyyyffffffffffffffffffffffff'

Input:  'co2[m]i2[te]'
Output: 'committee'