The Nightstar Zoo

perl -le "(1x++$_)!~/^(11+)\1+$/&&print while 1"

1) What does this do?
2) Do you understand it?
3) Can you explain how it works?
4) Were you able to do this without running it?

Enquiring minds want to knwo.

1. Got me.
2. No.
3. No.
4. Wha?

1) It prints out a list of prime numbers

2) It's an obfuscated code example, it's designed deliberately to be hard to read
and misleading

3) From left to right:

"perl"
the executable used to invoke PERL

"-le"
command line switches
'l' will put each "print" statement on it's own line
'e' is used to tell PERL that you want to execute the rest of the line as PERL code

"(1x++$_)"
this is obfuscated code that repeats the character '1' as many times as set in
the variable '$_', then it increments $_ by 1, for example when $_ is 5, (1x++$_)
would generate 11111 and then store 6 in $_

"!~"
This is a PERL regular expression operator for non-matches, it compares the
left-hand side of the operator to the expression on the right-hand side and
returns true if the LHS doesn't match the regexp on the RHS

"/^(11+)\1+$/"
This is the regular expression that determines whether or not the number is
prime based on the number of 1's generated

"&& print"
This prints out the value of $_ if everything to the left of the && returned a
true value (in this case, as long as the series of 1's doesn't match the above
pattern)

"while 1"
This creates an infinite loop around the line, PERL will continue to add 1 to $_
and check if it's prime until the script is terminated

4) No, I did not run it.

Perhaps not, but I bet you had help.

Yup...had access to some books.

You never stated that we couldn't use books or available resources.

Besides, I like a good challenge. It's like Mindtrap. You have to think.

No, I didn't. You glossed over one point that I would think is of algorithmic significance...

...But perl regexps are cryptic enough _without_ explanation, so I'll award credit.

Nice job.

Promise to do better next time.

This just confirms my belief that Perl is an icky, horrible language.

_________________
Scharr, scharr, verscharr das Gebein, grab es tief unten im Keller ein.
Später dann graben es andere aus, und nennen dein Haus das Knochenhaus.
Scharr, scharr, verscharr das Gebein, leg auch ihre weißen Schädel hinein.
Mit Beton gießt du es aus, das Fundament vom Knochenhaus.
Scharr, scharr, verscharr das Gebein, da ist noch Platz, da paßt noch wer rein.
Hier tobte sich der Teufel aus, unten im Keller im Knochenhaus.

And that some people have more time than they can profitably use.

jealous?

To be exact, the regex pattern

^(11+)\1+$

returns a large group if and only if there are exactly m>1 (one plus at least one) groups of n>1 1's (again, one plus at least one) from the start to the end of the line... so that m>1 * n>1 = m*n, so the number of 1's is composite if it matches.

Note that I know precisely jack about perl.

Vorn