Reverse letters

You are given a string. Write a script to reverse only the alphabetic characters in the string.

Examples

Example 1
Input: $str = "p-er?l"
Output: "l-re?p"

Example 2
Input: $str = "wee-k!L-y"
Output: "yLk-e!e-w"

Example 3
Input: $str = "_c-!h_all-en!g_e"
Output: "_e-!g_nel-la!h_c"

Analysis

Someone will no doubt come up with a one-liner, but my solution:

• Extracts the letters from the string as another string
• Reverses that string
• Builds the output string by substiting the next letter from the reversed string for each letter in the original string.

With regex based solution, I always need explanation otherwise you spend good amount of time to get your head around if it is a complex one. Here you even have DIY tool to test it as well. Great work.

Try it 

#!/usr/bin/perl

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

use v5.26;    # The Weekly Challenge - 2025-03-17
use utf8;     # Week 313 - task 2 - Reverse letters
use warnings; # Peter Campbell Smith
binmode STDOUT, ':utf8';

reverse_letters('p-er?l');
reverse_letters('wee-k!L-y');
reverse_letters('_c-!h_all-en!g_e');
reverse_letters('neerg:si:ssarg');

sub reverse_letters {
    
    my ($string, $letters, $reversed, $char, $i);
    
    # extract letters and reverse them
    $string = shift;
    $letters = $string;
    $letters =~ s|[^a-z]||gi;
    $letters = reverse($letters);
    
    # substitute a reversed letter for every letter in string
    for $char (split('', $string)) {
        $reversed .= $char =~ m|[a-z]|i ? substr($letters, $i ++, 1) : $char;
    }
    
    # report
    say qq[\nInput:  \$string   = '$string'];
    say qq[Output: \$reversed = '$reversed'];
}

10 lines of code

Output from script

Input:  $string   = 'p-er?l'
Output: $reversed = 'l-re?p'

Input:  $string   = 'wee-k!L-y'
Output: $reversed = 'yLk-e!e-w'

Input:  $string   = '_c-!h_all-en!g_e'
Output: $reversed = '_e-!g_nel-la!h_c'

Input:  $string   = 'neerg:si:ssarg'
Output: $reversed = 'grass:is:green'