Useful Uses of regex code expressions?

[kvale]

Back when I wrote perlretut , code expressions (?{code}) and (??{code}) were relatively new beasts in the regex menagerie. I thought they were clever and the variable backtracking was a little bit magical.

The trouble was, I couldn't come up with any examples where code expressions really nailed the problem. So for the tutorial I created a cute, but useless, example of matching Fibonacci strings:

# 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";

This prints

    It is a Fibonacci sequence
    Largest sequence matched was 5

Since then, I have written many regexes, but haven't come across any code that needed code expressions, as opposed to a combination of simpler regexes and perl code.

My question is: Has anyone found a good use for code expressions in their regex work?

-Mark