Perl rindex Function

last modified April 4, 2025

The Perl rindex function searches for a substring from the end of a string. It returns the position of the last occurrence or -1 if not found.

rindex is similar to index but searches right to left. It's useful for finding file extensions, parsing paths, or processing data from the end.

Basic rindex Usage

The simplest form of rindex takes a string and substring.

basic.pl

#!/usr/bin/perl

use strict;
use warnings;
use v5.34.0;

my $text = "Hello world, hello Perl";
my $pos = rindex($text, "hello");

print "Last 'hello' found at position: $pos\n";

This finds the last occurrence of "hello" in the string. Positions start at 0 from the string's beginning.

$ ./basic.pl
Last 'hello' found at position: 13

Finding File Extensions

rindex is perfect for locating file extensions by finding the last dot.

extension.pl

#!/usr/bin/perl

use strict;
use warnings;
use v5.34.0;

my $filename = "document.backup.txt";
my $dot_pos = rindex($filename, ".");

if ($dot_pos != -1) {
    my $ext = substr($filename, $dot_pos + 1);
    print "File extension: $ext\n";
} else {
    print "No extension found\n";
}

We use rindex to find the last dot, then substr to extract the extension. This handles multiple dots correctly.

$ ./extension.pl
File extension: txt

Searching with Position Limit

rindex can search up to a specified position from the start.

limit.pl

#!/usr/bin/perl

use strict;
use warnings;
use v5.34.0;

my $text = "abcXdefXghiXjkl";
my $pos = rindex($text, "X", 8);

print "Last 'X' before position 8: $pos\n";

This finds the last "X" occurring at or before position 8. The search starts from position 8 and moves left.

$ ./limit.pl
Last 'X' before position 8: 6

Processing Path Components

rindex helps extract directory paths from full filenames.

path.pl

#!/usr/bin/perl

use strict;
use warnings;
use v5.34.0;

my $path = "/home/user/docs/file.txt";
my $last_slash = rindex($path, "/");

if ($last_slash != -1) {
    my $dir = substr($path, 0, $last_slash);
    my $file = substr($path, $last_slash + 1);
    print "Directory: $dir\n";
    print "Filename: $file\n";
}

We find the last slash to separate directory and filename components. This technique works for both Unix and Windows paths.

$ ./path.pl
Directory: /home/user/docs
Filename: file.txt

Case-Insensitive Search

Combine rindex with lc for case-insensitive searches.

case.pl

#!/usr/bin/perl

use strict;
use warnings;
use v5.34.0;

my $text = "Hello World, hello Perl";
my $lc_text = lc($text);
my $pos = rindex($lc_text, "hello");

print "Last 'hello' (case-insensitive) at: $pos\n";
print "Original substring: ", substr($text, $pos, 5), "\n";

We convert the string to lowercase first, then search. The position matches the original string's case.

$ ./case.pl
Last 'hello' (case-insensitive) at: 13
Original substring: hello

Finding Multiple Occurrences

Use rindex in a loop to find all occurrences from the end.

multiple.pl

#!/usr/bin/perl

use strict;
use warnings;
use v5.34.0;

my $text = "abXcdXefXghXij";
my $sub = "X";
my $pos = length($text);

while (($pos = rindex($text, $sub, $pos - 1)) != -1) {
    print "Found '$sub' at position $pos\n";
    last if $pos == 0;  # Prevent infinite loop
}

We start from the end and move left with each iteration. The loop stops when no more matches are found.

$ ./multiple.pl
Found 'X' at position 9
Found 'X' at position 6
Found 'X' at position 3
Found 'X' at position 1

Comparing rindex and index

Demonstrating the difference between rindex and index.

compare.pl

#!/usr/bin/perl

use strict;
use warnings;
use v5.34.0;

my $text = "apple,banana,apple,cherry";
my $sub = "apple";

my $first_pos = index($text, $sub);
my $last_pos = rindex($text, $sub);

print "First '$sub' at: $first_pos\n";
print "Last '$sub' at: $last_pos\n";

index finds the first occurrence from the left, while rindex finds the last from the right.

$ ./compare.pl
First 'apple' at: 0
Last 'apple' at: 13

Best Practices

Check for -1: Always verify if rindex found a match.
Combine with substr: Extract portions after finding positions.
Use position limits: Constrain searches to relevant sections.
Consider performance: rindex is efficient even on large strings.

Source

Perl rindex Documentation

This tutorial covered Perl's rindex function with practical examples demonstrating its usage in common scenarios.

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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