I am seeking enlightenment to understand the behavior of the following program and to know what tools are available to understand/debug/display what is going on.

My test program:

use strict;
use warnings;
use Data::Dumper;

my @strings = qw(exception:tex exception:mex exception:mex asdf tex:ex
+ception:mex asdf exception:mex asdf asdf exception:mex);

foreach (1..4) {
    my @filtered = grep { /exception:/g && !/\Gtex/ } @strings;
    print Dumper(\@filtered);
}
[download]

produces the following output (perl 5.10.0 on CentOS 5.3):

$VAR1 = [
          'exception:mex',
          'exception:mex',
          'tex:exception:mex',
          'exception:mex',
          'exception:mex'
        ];
$VAR1 = [];
$VAR1 = [
          'exception:mex',
          'exception:mex',
          'tex:exception:mex',
          'exception:mex',
          'exception:mex'
        ];
$VAR1 = [];
[download]

I am expecting the same output from each iteration.

One way to get my expected output is to add "pos = 0;".

use strict;
use warnings;
use Data::Dumper;

my @strings = qw(exception:tex exception:mex exception:mex asdf tex:ex
+ception:mex asdf exception:mex asdf asdf exception:mex);

foreach (1..4) {
    my @filtered = grep { pos = 0; /exception:/g && !/\Gtex/ } @string
+s;
    print Dumper(\@filtered);
}
[download]

which produces

$VAR1 = [
          'exception:mex',
          'exception:mex',
          'tex:exception:mex',
          'exception:mex',
          'exception:mex'
        ];
$VAR1 = [
          'exception:mex',
          'exception:mex',
          'tex:exception:mex',
          'exception:mex',
          'exception:mex'
        ];
$VAR1 = [
          'exception:mex',
          'exception:mex',
          'tex:exception:mex',
          'exception:mex',
          'exception:mex'
        ];
$VAR1 = [
          'exception:mex',
          'exception:mex',
          'tex:exception:mex',
          'exception:mex',
          'exception:mex'
        ];
[download]

I don't understand the behavior and I don't know what tools are available to easily inspect the behavior. I have used gdb to debug RE code in the past, but it is a bit painful.

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