Perl Christmas Quiz 2009

[Mark Zealey]

> 7) Write a one line program that takes a non-negative integer as an
>  argument and prints the square root when the answer's an integer.
> 
> 
> 
> Restrictions: the perl line should be a regular expression.
> You are allowed to use the following functions/operators "x", "-",
>  "length", "print" plus any of the usual regular expression bestiary.

I'm pretty close but can't find a proper way to go to the previous capture 
group. eg the following works as much as you care to extend it:

("a" x $ARGV[0]) =~ /^
        (a)
        ( (a\g{-3})\g{-3} )?
        ( (a\g{-3})\g{-3} )?
        ( (a\g{-3})\g{-3} )?
        ( (a\g{-3})\g{-3} )?
        ( (a\g{-3})\g{-3} )?
        ( (a\g{-3})\g{-3} )?
    $
    (?{print length $+})
 /x;

However the obvious solution of using ( (a\g{-3})\g{-3} )* doesnt work as the 
capture buffers arn't updated on each match; and I can't manage to get the 
newer recursive syntax of (?-X) working properly. This would be worse anyway 
because then you could only go up to 50*50 before you get killed for deep 
recursion. The must be a way perhaps using the (?| syntax to get the capture 
buffers updated each time round?

Any thoughts?

Mark