Hi monks I have managed to create a function which returns a different result the second time you call it - with the same parameter. To cut things short, here it is: 
sub getChecksum($) {
        my $in = shift;
        my $check = 0;
        $in =~ m/(?:(.)(?{$check += ord($1);}))*^/;
        return $check;
}

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The function is supposed to return the sum of the decimal ASCII value of each character in the given string $in. Call it the first time (let's say, with the string foo), it returns 324, call it again, it returns 0. I then thought, that maybe the code in ?{} is not executed at all, but try replacing the regex with this: 
$in =~ m/(?:(.)(?{print "ASCII value of '$1' is: ".ord($1)."\n"; $chec
+k += ord($1);}))*^/;

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You'll see that even on the second call, it evaluates the code in ?{}, but does not increment $check. 
getChecksum called with: 'foo'
ASCII value of 'f' is: 102
ASCII value of 'o' is: 111
ASCII value of 'o' is: 111
will return '324'

getChecksum called with: 'foo'
ASCII value of 'f' is: 102
ASCII value of 'o' is: 111
ASCII value of 'o' is: 111
will return '0'

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Surely I could replace the regex with this: 
        my @chars = split( //, $in );
        foreach my $char (@chars) {
                $check += ord($char);
        }

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But who would want this when it should(?) work in the regex? ;-) I am running this on debian etch (perl v5.8.8). Anybody who has an explanation for this is allowed to run to the shop and buy himself a cookie :D

In reply to Weird behaviour with match-time code evaluation and backtracking by moensch

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