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.
#!/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.
#!/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.
#!/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.
#!/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.
#!/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.
#!/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.
#!/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
This tutorial covered Perl's substr function with practical
examples demonstrating its usage in common string manipulation scenarios.
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