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in reply to Re: Regexp substitution on variable-length ranges with embedded code? (updated)
in thread Regexp substitution on variable-length ranges with embedded code?

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

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(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))

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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~

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Re^3: Regexp substitution on variable-length ranges with embedded code?
by AnomalousMonk (Archbishop) on May 26, 2021 at 14:59 UTC
    ... I don't understand ... this line ...

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

    Is that double-eval'ed or executed? What is the '*F' referencing ...

    The embedded code is executed. 

          +----------------------+------ embedded Perl code
      |                      |
      v                      v
(?(?{ $run[-1] - $run[-2] != 1 }) (*F))

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    This is the "(?(*condition*)*yes-pattern*)" regex expression added with Perl version 5.10 (see Extended Patterns in perlre). In this case, the *condition* is the true/false result of evaluating the code. If true, the (*F) (a.k.a. (*FAIL)) backtracking control verb is executed and the match fails and backtracks to the most recent successfully matched substring: a sequence with incrementing values for $3.

    ... what is the '$^N' ...

    The $^N Perl special variable (see Variables related to regular expressions in perlvar) returns the value of the most recently closed capture group.

    Give a man a fish:  <%-{-{-{-<

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