Beefy Boxes and Bandwidth Generously Provided by pair Networks
There's more than one way to do things
 
PerlMonks  

japhy has met his match

by japhy (Canon)
on May 25, 2001 at 18:18 UTC ( #83316=perlmeditation: print w/ replies, xml ) Need Help??

I came up with the worst idea this morning, around 8:54 AM EDT.
(from #irc.linux.com::perl) <japhy> wow. <japhy> no, I just gave myself a difficult challenge. <evilgwynie> what is that? <japhy> $regex =~ $regex; <japhy> create a pattern that matches itself, and NOTHING ELSE <evilgwynie> hmm
Ugh. What did I get myself into?! I labored for an hour or so, and I've not been able to get anything concrete.

One large problem is the exponential increase of backslashes:

to match you need backslashes -------- -------- ----------- ^ \^ 1 \^ \\\^ 3 \\\^ \\\\\\\^ 7 \\\\\\\^ \\\\\\\\\\\\\\\^ 15
That pattern is, of course, 2**n - 1. It is super icky.

japhy -- Perl and Regex Hacker

Comment on japhy has met his match
Select or Download Code
Re: japhy has met his match
by Masem (Monsignor) on May 25, 2001 at 19:08 UTC
    As fuel for the fire, here's a regex that does match against itself, and only a small number of other cases besides itself. I know it doesn't satisify the "only itself" bit, but it could help get others thinking in the right direction:

    #Note that even with single quotes, '\\' is #interpolated to '\'; this doesn't break the regex, #as both sides of =~ will see only '\'. $regex = '^[24\\\\\[\]\{\}\^\$]{24}$'; $text = '2' x 24; # will also match, for example.

    Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain
      That's the type of approach I took. I try to beef it up, then, by using (?<=...) to require that the text used in the regex so far MUST be there. Oh, and if were to find a regex, it can't use $ -- it must use \z, since $ can match before a newline at the end of a string.

      japhy -- Perl and Regex Hacker
        Replacing $ with \z is trival:
        $regex = '^[z26\\\\\[\]\{\}\^\$]{26}\z';
        I've also worked out what the 'meta match' is for this expression (that is, $re2 below will only match $regex):
        $re2 = '^\^\\[z26\\\\\\\\\\\\\[\\\\\]\\\\\{\\\\\}\\\\\^\\\\\$\\]\\{26\ +\}\\\\z\z';
        So, the idea is your want, in the regex, to have $re2 attempt to match $regex, then have $regex left around that basically is a large character class with a {n} modifier (Thus, if we need to add any new characters like '(', ')', or '?', we work with $regex to add them, modify $re2 above, and we're set).

        However, I can't figure out how to do this part. I'm still thinking about it, but this is a rather interesting problem.


        Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain
solution to: Re: japhy has met his match
by japhy (Canon) on May 25, 2001 at 19:56 UTC
    Ok, updated AGAIN, thanks to MeowChow pointing out the obvious. ;)
    123456789_123456789_123456789_123456789_123456789_123456 $re = q{^.*(??{$&ne'^.*(??{'.substr($re,7,-4).'})\z'&&'(?!)'})\z};


    japhy -- Perl and Regex Hacker
      Close, but not quite yet:

      First, here's a 5.005 version that's the same thing:

      $regex = '^(.*)(?(?{my$x=substr$1,9,-4;$1ne"^(.*)(?(?{$x}))\\z"}))\z';
      And here's the stinger:
      $test = '^(.*)(?(?{my$japhy;my$x=substr$1,9,-4;$1ne"^(.*)(??{$x}))\\z +"}))\z';
      (Mind you, I'm playing with this on 5.005, and things that I don't expect to match are matching - I wonder if there's a bug in the RE engine at this point. However, I'm pretty sure that adding any text prior to "my$x..." will still match...)

      I think that you need to specify the exact length of the substr that you're looking at, which means to just simply do some character counting.


      Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain
      Am I missing something?
      print q|^.*(??{ })\z| =~ m|^.*(??{$&ne'^.*(??{'.substr($&,7,36).'})\z'})\z| # outputs 1
      Update to japhy's update: In that case, why not simply:
      $re = q{^.*(??{$&ne$re&&'(?!)'})\z};
         MeowChow                                   
                     s aamecha.s a..a\u$&owag.print
Re: japhy has met his match
by bikeNomad (Priest) on May 25, 2001 at 20:27 UTC
    I don't know what you mean by "and NOTHING ELSE", but wouldn't this do the trick?
    my $a = 'a'; print $a =~ $a; # prints 1
      No, because the regex 'a' also matches 'this is a line'. The idea was to only match itself, and no other string.

      The 15 year old, freshman programmer,
      Stephen Rawls
Re: japhy has met his match
by ozzy (Sexton) on May 26, 2001 at 04:36 UTC
    You have way to much time on your hands : )

    -= Ozzy =-
Re: japhy has met his match
by Zaxo (Archbishop) on May 26, 2001 at 10:53 UTC
    use Magic; tilt() if ($str =~ m/$Magic::selfish/ && ${str}abc !~ m/$Magic::selfish/); tilt() if ($Magic::selfish =~ m/$str/ && ${Magic::selfish}abc !~ m/$str/);
    Matching is a one-to-many operation. More succintly:
    die "42" if ${Magic::selfish}z !~ m/$Magic::selfish/;
    You're tilting at a windmill.

    After Compline,
    Zaxo

Log In?
Username:
Password:

What's my password?
Create A New User
Node Status?
node history
Node Type: perlmeditation [id://83316]
Approved by root
help
Chatterbox?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others having an uproarious good time at the Monastery: (11)
As of 2014-12-26 18:22 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?

    Is guessing a good strategy for surviving in the IT business?





    Results (174 votes), past polls