Just to make sure I understand the question, you're looking for a regex that does the equivalent of the following regex, but without (?{ }) or (??{ })? (Anything else that's not allowed?) Personally, I don't find this particular (??{ }) construct too terrible...

I played around a bit with recursion but then I was banging my head against the string 'abacbc', now I have to stop for today.

use warnings;
use strict;
use Algorithm::Combinatorics 'variations_with_repetition';
use Test::More;

my $re = qr{ \A
        ( .* )
        ( (??{ length $1 ? "[^".quotemeta($1)."]" : "." }) )
        ( (?: (?! \2 ) . )* )
    \z }msx;

for my $len (1..7) {
    my $iter = variations_with_repetition([qw/a b c/], $len);
    while ( my $c = $iter->next ) {
        my $str = join '', @$c;
        my $exp = do {
            my %h; $h{$_}++ for @$c;
            my %r = reverse %h;
            exists $r{1} };
        is $str=~$re, $exp, $str.($exp?' =~':' !~').' re'
            or diag explain
                [ \%-, map [ $-[$_], eval "\$$_", $+[$_] ], 1..@+ ];
    }
}

done_testing;

[download]

Update: A bit of research on variable-width lookbehind: Why is variable length lookahead implemented while lookbehind is not?, Re: perl regexp question excluding strings, Not-really variable length lookbehind

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

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