NAME

Debug::SawAmpersand - Perl extension querying sawampersand variable

SYNOPSIS

  use Debug::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: 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, 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.

SEE ALSO

perl(1).