NAME
Devel::SawAmpersand - Perl extension querying sawampersand variable
SYNOPSIS
use Devel::SawAmpersand qw(sawampersand);
sawampersand();
DESCRIPTION
There's a global variable in the perl source, called sawampersand. It
gets set to true in that moment in which the parser sees one of $`, $',
and $&. It never can be set to false again. Trying to set it to false
breaks the handling of the $`, $&, and $' completely.
If the global variable `sawampersand' is set to true, all subsequent RE
operations will be accompanied by massive in-memory copying, because
there is nobody in the perl source who could predict, when the
(necessary) copy for the ampersand family will be needed. So all
subsequent REs are considerable slower than necessary.
There are at least three impacts for developers:
* never use $& and friends in a library.
* Never "use English" in a library, because it contains the three bad
fellows.
* before you release a module or program, check if sawampersand is set by
any of the modules you use or require.
Workaround
Fortunately perl offers easy to use alternatives, that is
instead of this you can use this
$` of /pattern/ $1 of /(.*?)pattern/s
$& of /pattern/ $1 of /(pattern)/
$' of /pattern/ $+ of /pattern(.*)/s
In general, apply `/^(.*)(pattern)(.*)$/s' and use $1 for $`, $2 for $&
and $+ for $' ($+ is not dependent on the number of parens in the
original pattern). Note that the `/s' switch can alter the meaning of
`.' in your pattern.
Future Directions
From:
[email protected] (John Macdonald)
Date: Wed, 24 Dec 1997 10:49:56 -0500
How about adding an option to matches/substitutes that explicitly sets
$`, $&, and $'? When this new option is seen, sawampersand will be set
for the duration of the match and then turned off. That causes the
expense for unflagged matches to go back to normal and only flagged
matches incur the extra expense and they would be the ones that actually
need it. (Or does sawampersand need to affect the compilation of the
RE's too?)
AUTHOR
Andreas Koenig, special thanks to Johan Vromans and John Macdonald for
parts of the manpage and to Doug MacEachern for the FindAmpersand.
SEE ALSO
Devel::FindAmpersand, B::FindAmpersand
