# detects if a binary string 1101010010001... 
# has a Fibonacci spacing 0,1,1,2,3,5,... of the 1's

$x = "1101010010001000001";
$s0 = 0; $s1 = 1; # initial conditions
print "It is a Fibonacci sequence\n"
   if $x =~ /^1         # match an initial '1'
                 (
                    (??{'0' x $s0}) # match $s0 of '0'
                    1               # and then a '1'
                    (?{
                       $largest = $s0;   # largest seq so far
                       $s2 = $s1 + $s0;  # compute next term
                       $s0 = $s1;        # in Fibonacci sequence
                       $s1 = $s2;
                      })
                 )+   # repeat as needed
               $      # that is all there is
              /x;
print "Largest sequence matched was $largest\n";