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Devel::SawAmpersand
Perl extension querying PL_sawampersand variable

Devel::SawAmpersand - Perl extension querying PL_sawampersand variable


NAME

Devel::SawAmpersand - Perl extension querying PL_sawampersand variable


SYNOPSIS


  use Devel::SawAmpersand qw(sawampersand);

  sawampersand();


DESCRIPTION

This module provides one single function:

$bool = Devel::SawAmpersand::sawampersand()
Returns a true value if the compiled code has the C-level global variable PL_sawampersand set.

There's a global variable in the perl source, called PL_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 PL_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.

  • Don't ``use English'' in a library, because it contains the three bad fellows. Corollary: if you really want to use English, do it like so:
    
        use English qw( -no_match_vars ) ;

  • before you release a module or program, check if PL_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: jmm@elegant.com (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, PL_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 PL_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.


LICENSE

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

See http://www.perl.com/perl/misc/Artistic.html


SEE ALSO

Devel::FindAmpersand, B::FindAmpersand

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