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.

Try it 

#!/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'