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This is an excerpt from the latest version perlfaq6.pod, which
comes with the standard Perl distribution. These postings aim to
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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, "${^MATCH}", "${^PREMATCH}", and
"${^POSTMATCH}" 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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