Question

I have a strange problem in matching a pattern.

Consider the Perl code below

#!/usr/bin/perl -w

use strict;
my @Array = ("Hello|World","Good|Day");

function();
function();
function();

sub function 
{
  foreach my $pattern (@Array)  
  {
    $pattern =~ /(\w+)\|(\w+)/g;
    print $1."\n";
  }
    print "\n";
}

__END__

The output I expect should be

---

Hello
Good

Hello
Good

Hello
Good

---

But what I get is

Hello
Good

Use of uninitialized value $1 in concatenation (.) or string at D:\perlfiles\problem.pl li
ne 28.
Use of uninitialized value $1 in concatenation (.) or string at D:\perlfiles\problem.pl li
ne 28.

Hello
Good

What I observed was that the pattern matches alternatively.
Can someone explain me what is the problem regarding this code.
To fix this I changed the function subroutine to something like this:

sub function 
{
    my $string;
    foreach my $pattern (@Array)
    {
        $string .= $pattern."\n";
    }
    while ($string =~ m/(\w+)\|(\w+)/g)
    {
            print $1."\n";
    }
    print "\n";
}

Now I get the output as expected.

Answer 1

It is the global /g modifier that is at work. It remembers the position of the last pattern match. When it reaches the end of the string, it starts over.

Remove the /g modifier, and it will act as you expect.