Playing around with regexes (abusing them :) on the weekend I came across the following (on Perl 5.6.1, ActiveState Build 626). Executing the same regex on the same string in a loop multiple times yields different results for the first run and the remaining runs. I'm running on Win2K, but I don't think this plays a role here. The code:

#!/usr/bin/perl
use strict;
use warnings;
use re qw/eval /;

my $pattern = q/(.)(?{
        print ++$counts[0];
})^/;

my $line = 'ab';

for (0..2) {
    my @counts = (0);

    print "$_: ";
    # $pattern .= '(?=.)';
    $line =~ /$pattern/;
    print "; \@counts = (", join(', ', @counts), ")\n";
}
print "\@main::counts = (", join(', ', @::counts), ")\n";
[download]

This prints - apart from the warning about the last line:

0: 12; @counts = (2)
1: 34; @counts = (0)
2: 56; @counts = (0)
@main::counts = ()
[download]

which means, it works the first time as expected but the next times my @counts doesn't get modified by the regex. However, inside the regex the variable seems to retain its value from execution to execution.

When using a package variable by changing my @counts to our @counts the program works as expected and prints:

0: 12; @counts = (2)
1: 12; @counts = (2)
2: 12; @counts = (2)
@main::counts = (2)
[download]

When uncommenting the $pattern .= line (and going back to my) - effectively changing the pattern in every loop (remark: this does not effect the working of the regex!), the code also works as expected printing:

0: 12; @counts = (2)
1: 12; @counts = (2)
2: 12; @counts = (2)
@main::counts = ()
[download]

My question - is this a known bug? Is it a bug at all or might I have overlooked a (well) documented feature ;-) and how does this behave in other versions of perl?

-- Hofmator

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