Anyone who does anything significant with regular expressions learns that the variables $&, $' and $` imposes significant performance penalty on all regular expression matches. I have personally witnessed just how significant the use of these variables can be on performance (KinoSearch & Large Documents).

perlvar says under @- the following:

$`  is the same as substr($var, 0, $-[0])
$& is the same as substr($var, $-[0], $+[0] - $-[0])
$' is the same as substr($var, $+[0])
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So my question is why can't these variables be implemented this way behind the scenes. If I could get access to the last variable successfully matched against, for instance, I could write a Tie interface for these variables very easy.

I am guessing it has to do with keeping track of the last variable matched and handling cases where that variable is changed before reading $& (such as assignment or a s///).

This question is really out of curiosity about the internals of the perl regexp engine.

Thanks,