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my $max_weight = 10;
my %items      = (      # value, weight: positive integers
  'a'  => { v => 1, w => 2 },
  'b'  => { v => 11,  w => 9 },
);

           first half           second half
           $1  $2
attempt 1: v   vvvvvvvvvvv  --> ww + wwwwwwwww = 11, not enough w's!
backtrack
attempt 2: v   <empty>      --> ww = 2, enough w's for this, so accept
+ed

# used to be:
# my $re = sprintf "%s0\n(?=\n%s)\n", $left, $right;
...
my $re = sprintf qq[%s0\n(?{ print "trying " . Dumper \\%%+})\n(?=\n%s
+)\n],
    $left, $right;
print $re;

STRING:
vvvvvvvvvvvv0wwwwwwwwww
...
          'a' => '',
          'b' => 'vvvvvvvvvvv'
        };

STRING:
vvvvvvvvvvvv0wwwwwwwwww
...
          'a' => '',
          'b' => 'vvvvvvvvvvv'
        };

^
(?<a>(?:v)?)
(?<b>(?:vvvvvvvvvvv)?)
v*
0

(?<a>(?:v)?)
(?<b>(?:vvvvvvvvvvv)?)
0