Miker has asked for the wisdom of the Perl Monks concerning the following question:

Why does this run?
#!/usr/bin/perl $words = "This%is%Devious"; $words =~ s/%/ /eg; $words =~ s/ /%/eg; } print "$words\n";

If you remove the lone right bracket it will not run, but I can't figure out why this is. Any illumination would be much obliged:)

Replies are listed 'Best First'.
Re: The Lone Right Bracket
by turnstep (Parson) on Jul 26, 2000 at 09:13 UTC
    Here's a simplified test case. For added fun, it also works with @, $ and *:
    #!perl s//%/e; }

    Eval is actually trying to use the surrounding code to try and finish it's evaluation. This was fixed. Recent version of perl (e.g. 5.6) will give this warning when the code above is run:

    Bad evalled substitution pattern at line X

    Looking at perl5004delta, we read that:

    Perl (version 5) used to determine the value of EXPR inconsistently, sometimes incorrectly using the surrounding context for the determination. Now, the value of EXPR (before being parsed by eval) is always determined in a scalar context.

    Unfortunately, each version keeps removing some of these little "features" that make for great obfuscated code. :)

    Here's some fun code to try out if you have an "older" version of perl (older is in quotes because it hasn't been fixed *that* recently):

    #!perl $foo = "ABC"; print "($foo)\n"; $foo =~ s/B/*/e; } print "($foo)\n";

    Running this script produces this rather odd output:

    (ABC) (A*main::} C)

    In other words, eval tried so hard to evaluate something, it regarded the * as a typeglob, and created something unusual, to say the least. :)

      Thanks for the insight, all the responses give me a lot to look into. To be honest the way I came across the oddity was late night typing but it seems that Perl mistakes are even beter learning experiences than everyday ones:)
Re: The Lone Right Bracket
by btrott (Parson) on Jul 26, 2000 at 08:46 UTC
    Well, it breaks because the /e modifier means, evaluate the right-hand side of this substitution as Perl code. And that's not valid Perl code. But you may have already known that.

    Did you try running it through B::Deparse? This is what you get:

    $words = 'This%is%Devious'; $words =~ s/%/do { () }/eg; $words =~ s/ /do { %} }\n/eg; print "$words\n";
    Of course, this isn't valid code either, nor is the original with the ending curly brace. :) But apparently you're tricking the parser into recognizing it as valid. Very odd...

    Some wonderful opportunities for obfuscation may lie down this path, it seems. :)

      How exactly do I run a program through B::Deparse? Is there a command line flag?