comment on

I just remembered that Perl supports recursive regexes and relative back-references, so you might not be limited to a hard limit.

Unfortunately I don't have the time to play with it.

FWIW if using s///g was allowed you just needed a pattern which repeatedly deleted one character of each group, iff exactly one of them is unique.

Your criterion was met, if the string is empty afterwards.

Cheers Rolf
_{(addicted to the Perl Programming Language and ☆☆☆☆ :)

Je suis Charlie!}

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