Perl reverse Function

last modified April 4, 2025

The Perl reverse function reverses the order of elements in a list or the characters in a string. Its behavior depends on the context in which it's used.

In list context, reverse reverses the order of list elements. In scalar context with a string, it reverses the character order. It doesn't modify the original data.

Basic String Reversal

The simplest use of reverse is to reverse a string in scalar context.

string_reverse.pl

#!/usr/bin/perl

use strict;
use warnings;
use v5.34.0;

my $text = "Hello World";
my $reversed = reverse $text;

print "Original: $text\n";
print "Reversed: $reversed\n";

This example shows string reversal in scalar context. The reverse function returns a new string with characters in reverse order.

$ ./string_reverse.pl
Original: Hello World
Reversed: dlroW olleH

List Reversal

In list context, reverse reverses the order of elements.

list_reverse.pl

#!/usr/bin/perl

use strict;
use warnings;
use v5.34.0;

my @numbers = (1..5);
my @reversed = reverse @numbers;

print "Original: @numbers\n";
print "Reversed: @reversed\n";

This demonstrates list reversal. The function returns a new array with elements in reverse order, leaving the original array unchanged.

$ ./list_reverse.pl
Original: 1 2 3 4 5
Reversed: 5 4 3 2 1

Reversing a String in List Context

The behavior changes when using reverse on a string in list context.

string_list_context.pl

#!/usr/bin/perl

use strict;
use warnings;
use v5.34.0;

my $text = "Hello";
my @chars = reverse $text;

print "Original: $text\n";
print "Reversed as list: @chars\n";

In list context, the string is treated as a list of characters. The function returns the reversed list, not a reversed string.

$ ./string_list_context.pl
Original: Hello
Reversed as list: o l l e H

Reversing Hash Keys

reverse can help process hash keys in reverse order.

hash_reverse.pl

#!/usr/bin/perl

use strict;
use warnings;
use v5.34.0;

my %fruit = (
    apple => 3,
    banana => 7,
    cherry => 2
);

my @reversed_keys = reverse keys %fruit;

print "Original order: ", join(', ', keys %fruit), "\n";
print "Reversed order: ", join(', ', @reversed_keys), "\n";

This example reverses the order of hash keys. Note that hash key order is normally arbitrary in Perl.

$ ./hash_reverse.pl
Original order: banana, apple, cherry
Reversed order: cherry, apple, banana

Combining with Other Functions

reverse is often combined with other list operations.

combined.pl

#!/usr/bin/perl

use strict;
use warnings;
use v5.34.0;

my @words = qw(apple banana cherry date elderberry);
my @sorted_reversed = reverse sort @words;

print "Original: @words\n";
print "Sorted and reversed: @sorted_reversed\n";

Here we first sort the list alphabetically, then reverse the sorted order. This shows how reverse can be part of a processing pipeline.

$ ./combined.pl
Original: apple banana cherry date elderberry
Sorted and reversed: elderberry date cherry banana apple

Reversing Lines from a File

reverse can process file contents in reverse order.

file_reverse.pl

#!/usr/bin/perl

use strict;
use warnings;
use v5.34.0;

open(my $fh, '<', 'data.txt') or die "Can't open file: $!";
my @lines = <$fh>;
close($fh);

my @reversed_lines = reverse @lines;

print "Original first line: $lines[0]";
print "Reversed first line: $reversed_lines[0]";

This script reads a file into an array, then reverses the line order. The last line becomes first, and vice versa.

Creating a Palindrome Checker

reverse can help identify palindromic strings.

palindrome.pl

#!/usr/bin/perl

use strict;
use warnings;
use v5.34.0;

sub is_palindrome {
    my $word = lc shift;
    $word =~ s/\W//g;  # Remove non-word characters
    return $word eq reverse $word;
}

print "Enter a word: ";
my $input = <STDIN>;
chomp $input;

if (is_palindrome($input)) {
    print "'$input' is a palindrome\n";
} else {
    print "'$input' is not a palindrome\n";
}

This example checks if a word reads the same forwards and backwards. We clean the input first to handle punctuation and case sensitivity.

$ ./palindrome.pl
Enter a word: Racecar
'Racecar' is a palindrome

Best Practices

Context awareness: Remember scalar vs list context differences.
Original unchanged: reverse returns new data, doesn't modify original.
Combine with join: Use join to convert reversed lists back to strings.
Efficiency: Avoid unnecessary reversals in large data processing.

Source

Perl reverse Documentation

This tutorial covered Perl's reverse function with practical examples demonstrating its usage with strings, lists, and files.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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