Re^2: Regexp substitution on variable-length ranges with embedded code?

[Polyglot]

Your solution works for me, with the addition of:

use re 'eval';

(I'm on Perl 5.12.)

...but I don't understand it. Specifically, this line is the most difficult one for me:

(?(?{ $run[-1] - $run[-2] != 1 }) (*F))

Is that double-eval'ed or executed? What is the '*F' referencing, and what is the '$^N' from the lines above? I've never seen this kind of regex before.