FAQ 6.19 Why does using $&, $`, or $' slow my program down?

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6.19: Why does using $&, $`, or $' slow my program down?

    (contributed by Anno Siegel)

    Once Perl sees that you need one of these variables anywhere in the
    program, it provides them on each and every pattern match. That means
    that on every pattern match the entire string will be copied, part of it
    to $`, part to $&, and part to $'. Thus the penalty is most severe with
    long strings and patterns that match often. Avoid $&, $', and $` if you
    can, but if you can't, once you've used them at all, use them at will
    because you've already paid the price. Remember that some algorithms
    really appreciate them. As of the 5.005 release, the $& variable is no
    longer "expensive" the way the other two are.

    Since Perl 5.6.1 the special variables @- and @+ can functionally
    replace $`, $& and $'. These arrays contain pointers to the beginning
    and end of each match (see perlvar for the full story), so they give you
    essentially the same information, but without the risk of excessive
    string copying.

    Perl 5.10 added three specials, "$", "$", and
    "$" to do the same job but without the global performance
    penalty. Perl 5.10 only sets these variables if you compile or execute
    the regular expression with the "/p" modifier.


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