> Just to make sure I understand the question, you're looking for a regex that does the equivalent of the following regex,

Yes, even without testing this looks right, your embedded Perl excludes all former characters in a character class.*

> Personally, I don't find this particular (??{ }) construct too terrible...

It's a theoretical question, I wouldn't use a regex anyway. It's about exploring the limits.

> but without (?{ }) or (??{ })? (Anything else that's not allowed?) 

Basically yes.

The less cheating the better, ie a portable solution would be perfect. (Bigly doubt)

Using implementation details or experimental features less so. (Sad )

> Update: A bit of research on variable-width lookbehind:

Regarding the older discussions:

I don't see how \K could be applied here, the perldocs are IMHO wrong to claim it was generally allowing variable length look behinds.

IMHO does \K only apply for s/// .

Cheers Rolf
(addicted to the Perl Programming Language and ☆☆☆☆ :)
Je suis Charlie!

*) that's the approach I would try to implement with recursive patterns

In reply to Re^2: Regex: matching character which happens exactly once by LanX
in thread Regex: matching character which happens exactly once by LanX

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