Reverse prefix

You are given a string, $str and a character in the given string, $char. Write a script to reverse the prefix upto the first occurrence of the given $char in $str and return the new string.

Examples

Example 1
Input: $str = 'programming', $char = 'g'
Output: 'gorpramming'
Reverse of prefix 'prog' is 'gorp'.

Example 2
Input: $str = 'hello', $char = 'h'
Output: 'hello'

Example 3
Input: $str = 'abcdefghij', $char = 'h'
Output: 'hgfedcbaij'

Example 4
Input: $str = 'reverse', $char = 's'
Output: 'srevere'

Example 5
Input: $str = 'perl', $char = 'r'
Output: 'repl'

Analysis

The essence of my solution is this:

if ($string =~ m|(.*?)$char(.*)|) {
	$result = $char . reverse($1) . $2;
}

That would suffice except for the case where $char doesn’t occur in $string. Of course the task statement excludes that possibility, but I feel it is prudent to check.

This is a practical, well-documented technical blog post that demonstrates a regex-focused approach to problem-solving with strong emphasis on robustness and error handling. Peter provides clean, working solutions with comprehensive examples and thoughtful edge case consideration.

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

Try it 

Try running the script with any input:

Your $str =

example: metpesber

Your $char =

example: s

⤡

#!/usr/bin/perl

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

use v5.26;    # The Weekly Challenge - 2025-09-29
use utf8;     # Week 341 - task 2 - Reverse prefix
use warnings; # Peter Campbell Smith
binmode STDOUT, ':utf8';
use Encode;

reverse_prefix('programming', 'g');
reverse_prefix('hello', 'h');
reverse_prefix('goodbye', 'e');
reverse_prefix('noitulos', 's');
reverse_prefix('conundrum', 'z');

sub reverse_prefix {
    
    my ($string, $char, $result);
    
    # initialise
    ($string, $char) = @_;
    
    # do as instructed
    if ($string =~ m|(.*?)$char(.*)|) {
        $result = qq['$char] . reverse($1) . qq[$2'];
        
    # non-compliant data!   
    } else {
        $result = qq[There is no '$char' in '$string'];
    }
        
    say qq[\nInput:  \$string = '$string', \$char = '$char'];
    say qq[Output: $result];
}

9 lines of code

Output from script

Input:  $string = 'programming', $char = 'g'
Output: 'gorpramming'

Input:  $string = 'hello', $char = 'h'
Output: 'hello'

Input:  $string = 'goodbye', $char = 'e'
Output: 'eybdoog'

Input:  $string = 'noitulos', $char = 's'
Output: 'solution'

Input:  $string = 'conundrum', $char = 'z'
Output: There is no 'z' in 'conundrum'