A co-worker recently added some modules to a large perl program that used $&, $' and $` (a.k.a. the "naughty match variables"). I know these add a large performance penalty for all regular expressions in the program, so I removed all the uses. Then I tried all three of the methods in Mastering Regular Expressions, second edition, p. 358, "How to Check Whether Your Code is Tainted by $&".

Only the last method on that page works for perl 5.8.0. The "-Mre=debug" does not show either 'Enabling $`, $&, $' support' or 'Omitting $`, $&, $' support' any more.

The Devel::SawAmpersand doesn't work either. It gives false positives on trivial programs that don't have the "naughty match variables".

Here is the subroutine that actually worked: 

use strict;
use Time::HiRes;

sub CheckNaughtiness ()
{
   my $text = 'x' x 10_000;

   my $start = Time::HiRes::time();
   for (my $i = 0; $i < 5_000; $i++) { }
   my $overhead = Time::HiRes::time() - $start;

   $start = Time::HiRes::time();
   for (my $i = 0; $i < 5_000; $i++) { $text =~ m/^/}
   my $delta = Time::HiRes::time() - $start;

   printf "It seems your code is %s (overhead=%.2f, delta=%.2f)\n",
      ($delta > $overhead*5) ? "naughty" : "clean", $overhead, $delta;
}

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To my great surprise, when I traced it out I found two CPAN modules (so far) that we use are also tainted in this way: Printer and Math::MatrixReal. I have sent mail to the maintainers of these modules pointing out the issue.

It makes me wonder how many other CPAN modules are tainted with "naughty match variables". Another way to get tainted is to do: 

# Don't do this:
use English;

# Do this instead:
use English qw( -no_match_vars );

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Has anyone else noticed this problem? Should there be a general check for "naughty match variables" for code submitted to CPAN?

In reply to Naughty match variables in CPAN? by tall_man

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