Perl pos Function

last modified April 4, 2025

The Perl pos function tracks and manipulates the position in a string where the next regex match will start. It's essential for advanced string parsing and iterative matching operations.

pos works with the /g regex modifier to enable sequential matching. It can both retrieve and set the current match position. The position is zero-based, counting from the start of the string.

Basic pos Usage

The simplest way to use pos is to check the current position.

basic.pl

#!/usr/bin/perl

use strict;
use warnings;
use v5.34.0;

my $text = "Hello world";
$text =~ /Hello/g;

print "Current position: ", pos($text), "\n";

We match "Hello" in the string and then check the position. The position after matching will be right after the matched substring.

$ ./basic.pl
Current position: 5

Iterative Matching with pos

pos is commonly used with /g for iterative matching.

iterate.pl

#!/usr/bin/perl

use strict;
use warnings;
use v5.34.0;

my $text = "a1 b2 c3 d4";
while ($text =~ /(\w)(\d)/g) {
    print "Found $1$2 at position ", pos($text), "\n";
}

This script finds all letter-number pairs in the string. The pos function shows where the next match will start after each iteration.

$ ./iterate.pl
Found a1 at position 2
Found b2 at position 5
Found c3 at position 8
Found d4 at position 11

Manually Setting pos

You can manually set the position to control where matching starts.

setpos.pl

#!/usr/bin/perl

use strict;
use warnings;
use v5.34.0;

my $text = "abc123xyz789";
pos($text) = 3;  # Skip first 3 characters

if ($text =~ /\d+/g) {
    print "First number found: $& at position ", pos($text), "\n";
}

We set the position to skip the first 3 letters. The regex then matches the first number sequence starting from that position.

$ ./setpos.pl
First number found: 123 at position 6

pos with Capture Groups

pos works with capture groups to track complex matches.

capture.pl

#!/usr/bin/perl

use strict;
use warnings;
use v5.34.0;

my $text = "Name: John, Age: 30, City: NY";
while ($text =~ /(\w+):\s*([^,]+)/g) {
    print "$1 = $2 (ends at ", pos($text), ")\n";
}

This script extracts key-value pairs separated by colons. The position after each match shows where the next search will begin.

$ ./capture.pl
Name = John (ends at 11)
Age = 30 (ends at 20)
City = NY (ends at 28)

Resetting pos

You can reset the position to start matching from the beginning again.

reset.pl

#!/usr/bin/perl

use strict;
use warnings;
use v5.34.0;

my $text = "apple banana cherry";
$text =~ /banana/g;

print "After first match: ", pos($text), "\n";

pos($text) = 0;  # Reset position
$text =~ /apple/g;

print "After reset and second match: ", pos($text), "\n";

We match "banana", then reset the position to match "apple" from the start. Resetting pos allows reusing the same string for new searches.

$ ./reset.pl
After first match: 12
After reset and second match: 5

pos with Unicode Strings

pos handles Unicode strings correctly when using appropriate settings.

unicode.pl

#!/usr/bin/perl

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

my $text = "café ☕ museum";
$text =~ /é/g;

print "Position after café: ", pos($text), "\n";

$text =~ /☕/g;
print "Position after coffee: ", pos($text), "\n";

This demonstrates pos working with Unicode characters. The position advances correctly for multi-byte characters when using UTF-8.

$ ./unicode.pl
Position after café: 4
Position after coffee: 7

Advanced pos Manipulation

pos can be used creatively for complex parsing tasks.

advanced.pl

#!/usr/bin/perl

use strict;
use warnings;
use v5.34.0;

my $text = "data:123-456-789;info:abc-def-ghi;";
my %data;

while ($text =~ /(\w+):([^;]+)/g) {
    my $key = $1;
    my $values = $2;
    my @values = split /-/, $values;
    $data{$key} = \@values;
    pos($text)++;  # Skip semicolon
}

use Data::Dumper;
print Dumper \%data;

This advanced example extracts structured data from a string. We manually adjust pos to skip semicolons between key-value pairs.

$ ./advanced.pl
$VAR1 = {
          'info' => [
                      'abc',
                      'def',
                      'ghi'
                    ],
          'data' => [
                      '123',
                      '456',
                      '789'
                    ]
        };

Best Practices

Use with /g modifier: pos primarily works with global regex matching.
Check definedness: Verify pos is defined before using its value.
Reset when needed: Clear pos with undef to restart matching.
Combine with substr: Use substr with pos for precise extraction.
Watch Unicode: Ensure proper encoding for accurate position tracking.

Source

Perl pos Documentation

This tutorial covered Perl's pos function with practical examples demonstrating its usage in string manipulation and parsing.

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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