Perl substr Function

last modified April 4, 2025

The Perl substr function extracts or replaces portions of strings. It’s a versatile tool for string manipulation with several operation modes.

substr can work as an lvalue (modifiable) or return a substring. It handles both positive and negative offsets for flexible string access.

Basic substr Usage

The simplest form extracts a substring starting at a position with a length.

basic.pl

#!/usr/bin/perl

use strict; use warnings; use v5.34.0;

my $text = “Hello, Perl world!”; my $sub = substr($text, 7, 4);

print “Original: $text\n”; print “Substring: ‘$sub’\n”;

We extract a 4-character substring starting at position 7 (0-based index). The function returns “Perl” without modifying the original string.

$ ./basic.pl Original: Hello, Perl world! Substring: ‘Perl’

Negative Offset

Negative offsets count from the end of the string, useful for suffix access.

negative.pl

#!/usr/bin/perl

use strict; use warnings; use v5.34.0;

my $filename = “document.txt”; my $extension = substr($filename, -3);

print “Filename: $filename\n”; print “Extension: ‘$extension’\n”;

Using -3 as offset starts counting from the string’s end. This extracts the last 3 characters, commonly used for file extensions.

$ ./negative.pl Filename: document.txt Extension: ’txt’

Omitting Length

When length is omitted, substr returns all remaining characters.

length_omit.pl

#!/usr/bin/perl

use strict; use warnings; use v5.34.0;

my $text = “The quick brown fox”; my $rest = substr($text, 10);

print “Original: $text\n”; print “From position 10: ‘$rest’\n”;

Starting at position 10, we get all remaining characters without specifying length. This is convenient for splitting strings at known positions.

$ ./length_omit.pl Original: The quick brown fox From position 10: ‘brown fox’

String Replacement

substr can modify strings when used as an lvalue.

replace.pl

#!/usr/bin/perl

use strict; use warnings; use v5.34.0;

my $text = “I like apples”; substr($text, 7, 6) = “oranges”;

print “Modified string: $text\n”;

We replace 6 characters starting at position 7 with “oranges”. The original string is modified directly through the lvalue assignment.

$ ./replace.pl Modified string: I like oranges

Inserting Strings

Using length 0 with replacement inserts without removing characters.

insert.pl

#!/usr/bin/perl

use strict; use warnings; use v5.34.0;

my $text = “Hello world!”; substr($text, 6, 0) = “beautiful “;

print “Modified string: $text\n”;

By specifying length 0, we insert “beautiful " at position 6 without removing any existing characters. This performs a pure insertion.

$ ./insert.pl Modified string: Hello beautiful world!

Combining with Regular Expressions

substr can work with regex matches for precise string operations.

regex.pl

#!/usr/bin/perl

use strict; use warnings; use v5.34.0;

my $text = “Date: 2023-04-15”; $text =~ /(\d{4}-\d{2}-\d{2})/;

my $date = substr($text, $-[1], $+[1] - $-[1]); print “Extracted date: $date\n”;

We use regex match variables $-[1] and $+[1] to get the start and end positions of the matched date. substr then extracts exactly this portion.

$ ./regex.pl Extracted date: 2023-04-15

Multi-byte Character Handling

For Unicode strings, special consideration is needed for character boundaries.

unicode.pl

#!/usr/bin/perl

use strict; use warnings; use v5.34.0; use utf8;

my $text = “日本語のテキスト”; my $sub = substr($text, 3, 3);

print “Original: $text\n”; print “Substring: $sub\n”;

With UTF-8 strings, positions refer to characters, not bytes. The example extracts 3 Japanese characters starting from position 3.

$ ./unicode.pl Original: 日本語のテキスト Substring: のテキ

Best Practices

• Check bounds: Verify positions are within string length.

• Use negative offsets: For convenient end-relative access.

• Document complex operations: When combining multiple features.

• Consider Unicode: Be aware of multi-byte character handling.

Source

Perl substr Documentation

This tutorial covered Perl’s substr function with practical examples demonstrating its usage in common string manipulation 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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