$Test_String = <STDIN> ;
if($Test_String =~ /$Regex_Pattern/){
    print "true";
} else {
    print "false";
}
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The top 10 contestants (most points + shortest time) won a T-shirt. Once I realised there were more then 10 people with the full score, I knew I wasn't getting one, but I still wanted to get the full score.

But I had no idea how to solve one of the tasks: the input was a long string of zeroes and ones. Your regex had to recognise whether the string encoded two binary numbers in the following way: when you reversed the string and extracted the odd digits, you got a number three times greater than the one built from the remaining digits. For example,

1110110001 => 00111, 10101
                  7     21
              3 * 7  =  21, accept!
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I wrote a short script to generate some examples, but I wasn't able to find the pattern. Moreover, the regex couldn't have more than 40 characters!

Then I remembered Perl has a way to run code in a regex: the (?{...}) pattern. I read the relevant perlre section several times and tried something like the following:

use bigint;

sub check {
    my ($bin, $three) = ('0b') x 2;
    my $s = $_;
    while ($s) {
        $three .= chop $s;
        $bin   .= chop $s;
    }
    return oct $three == 3 * oct $bin
}

$Regex_Pattern = qr/(?{ check() })/;
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The problem here is that (?{...}) always matches. Fortunately, you can use the code pattern as the condition in

(?(condition)yes-pattern|no-pattern)
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As the yes-pattern, I used ^ which always matches, and (*FAIL) for the no-pattern:

$Regex_Pattern = qr/^(?(?{ check() }) ^ | (*FAIL) )/x;
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The qr adds some characters, so to be sure I don't overflow, I renamed the subroutine to c and golfed the solution to

'^(?(?{c()})^|(*F))'
[download]

I got the full score! Would you call such a solution cheating? On one hand, I see that's not what the organisers wanted me to do, on the other hand, that's what Perl regular expressions give you. In fact, with the "start or fail" pattern, I can solve almost any problem with a regular expression!

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]