Task — Decrypt string

You are given a string formed by digits and '#'. Write a script to map the given string to English lowercase characters following the given rules:

• Characters 'a' to 'i' are represented by '1' to '9' respectively.
• Characters 'j' to 'z' are represented by '10#' to '26#' respectively.

Examples

Example 1
Input: $str = "10#11#12"
Output: "jkab"
10# -> j
11# -> k
1   -> a
2   -> b

Example 2
Input: $str = "1326#"
Output: "acz"
1   -> a
3   -> c
26# -> z

Example 3
Input: $str = "25#24#123"
Output: "yxabc"
25# -> y
24# -> x
1   -> a
2   -> b
3   -> c

Example 4
Input: $str = "20#5"
Output: "te"
20# -> t
5   -> e

Example 5
Input: $str = "1910#26#"
Output: "aijz"
1   -> a
9   -> i
10# -> j
26# -> z

Analysis

This is easily done with two regexes. The 'e' modifier - like s|x|$y|e - is slower than a fixed regex as it has to be parsed at run time each time it's encountered, but for a task like this I doubt that is significant.

Equally, I'd avoid using this if you're working for MI5, but I expect you knew that.

Try it 

#!/usr/bin/perl

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

use v5.26;    # The Weekly Challenge - 2026-03-09
use utf8;     # Week 364 - task 1 - Decrypt string
use warnings; # Peter Campbell Smith
binmode STDOUT, ':utf8';
use Encode;

decrypt_string('10#11#12');
decrypt_string('1326#');
decrypt_string('25#24#123');
decrypt_string('20#5');
decrypt_string('1910#26#');

sub decrypt_string {
    
    my ($string, $base);
    
    # initialise
    $string = $_[0];
    say qq[\nInput:  '$string'];
    $base = ord('a') - 1;
    
    # apply rules
    $string =~ s|([12][0-9])#|chr($base + $1)|ge;
    $string =~ s|([1-9])|chr($base + $1)|ge;
    
    say qq[Output: '$string'];
}

last updated 2026-03-09 — 8 lines of code

Output

Input:  '10#11#12'
Output: 'jkab'

Input:  '1326#'
Output: 'acz'

Input:  '25#24#123'
Output: 'yxabc'

Input:  '20#5'
Output: 'te'

Input:  '1910#26#'
Output: 'aijz'