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Devel::SawAmpersand - Perl extension querying PL_sawampersand variable


  use Devel::SawAmpersand qw(sawampersand);



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. Use /p if you have perl 5.10 or later.

  • 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.


Fortunately, perl offers easy to use alternatives. If you have perl 5.10 or later, you can use the /p match operator flag to turn on per-match variables that do the same thing:

       instead of this              you can use this

     $`   of   /pattern/          ${^PREMATCH}  of  /pattern/p
     $&   of   /pattern/          ${^MATCH}     of  /pattern/p
     $'   of   /pattern/          ${^POSTMATCH} of  /pattern/p

If you are using an older perl, you can use these workarounds:

       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.


Andreas Koenig, special thanks to Johan Vromans, John Macdonald, and brian d foy for parts of the manpage and to Doug MacEachern for the FindAmpersand.


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



Devel::FindAmpersand, B::FindAmpersand