P
PerlFAQ Server
This is an excerpt from the latest version perlfaq6.pod, which
comes with the standard Perl distribution. These postings aim to
reduce the number of repeated questions as well as allow the community
to review and update the answers. The latest version of the complete
perlfaq is at http://faq.perl.org .
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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.
--------------------------------------------------------------------
The perlfaq-workers, a group of volunteers, maintain the perlfaq. They
are not necessarily experts in every domain where Perl might show up,
so please include as much information as possible and relevant in any
corrections. The perlfaq-workers also don't have access to every
operating system or platform, so please include relevant details for
corrections to examples that do not work on particular platforms.
Working code is greatly appreciated.
If you'd like to help maintain the perlfaq, see the details in
perlfaq.pod.
comes with the standard Perl distribution. These postings aim to
reduce the number of repeated questions as well as allow the community
to review and update the answers. The latest version of the complete
perlfaq is at http://faq.perl.org .
--------------------------------------------------------------------
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.
--------------------------------------------------------------------
The perlfaq-workers, a group of volunteers, maintain the perlfaq. They
are not necessarily experts in every domain where Perl might show up,
so please include as much information as possible and relevant in any
corrections. The perlfaq-workers also don't have access to every
operating system or platform, so please include relevant details for
corrections to examples that do not work on particular platforms.
Working code is greatly appreciated.
If you'd like to help maintain the perlfaq, see the details in
perlfaq.pod.