"Only perl can parse Perl"

        $u = blub / 3;          # Is this a comment? /;

• Where did blub come from?

        package Blub;
        use Astro::MoonPhase;

        sub import {
          my $caller = caller;
          my ($phase) = phase(time());
          if (0.4 < $phase && $phase < 0.6) {
            *{$caller . "::blub"} = sub () { 1 };
          } else {
            *{$caller . "::blub"} = sub ($) { $_[0] };
          }
        }

• This program parses differently when the moon is full

  • So to fully parse all Perl programs, you must be able to determine the phase of the moon

• And that's why only perl can parse Perl

• Nevertheless, filters can do reasonably well in practice

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Copyright © 2003 M. J. Dominus