Your solution works for me, with the addition of: 
use re 'eval';

[download]
(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))

[download]

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.

Blessings,

~Polyglot~

In reply to Re^2: Regexp substitution on variable-length ranges with embedded code? by Polyglot
in thread Regexp substitution on variable-length ranges with embedded code? by Polyglot

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