my $re2 = qr{
	(?<char> . )
	(?= (?<lookback>
		(?<=
			(?! \g{char} ) .
			(?=(?&lookback)) .
		|
			(?= \g{char} ) . .
		)
	) )
}msx;
my $re2_short = qr
	/(.)(?=((?<=(?!\1).(?=(?-1)).|(?=\1)..)))/sx;