/(.)(?!((?<=(?!\1).(?=(?2)).|(?=\1)..))|.*\1)/s

##</code><code>##

qr{
	(?<cur> . )
	#(?(?= (?&lookback) | .* \g{cur} )
	#	(?{print "cur($+{cur}) seen before or after\n"})
	#	(*FAIL)
	#)
	# simpler:
	(?! (?&lookback) | .* \g{cur} )
	(?{ print "post: $-[0]<$&>$+[0]\n" })
	(?(DEFINE) (?<lookback>
		(?<=
			(?<prev>  (?! \g{cur} ) . )
			(?{ print "  prev: <$+{prev}> / $-[0]<".($&//'-').">$+[0]\n" })
			(?=(?&lookback)) .
		|
			(?= \g{cur} ) . .
		)
	) )
}msx