/^(?(?=((?(3)(?(?!.*\3)(?(?!((?<=(?!\3).(?=(?2)).
|(?=\3)..)))(*F))))(?(?=.)(?=(.)(?1)))))(*F))/sx

##</code><code>##

qr{
	(?(DEFINE) (?<loopfwd>
		(?{ print "pre:  $-[0]<$&>$+[0] pos:".pos."\n" })
		
		(?(<cur>) # we have a current character to inspect
			(?{ print "*CUR: <$+{cur}> / $-[0]<$&>$+[0] pos:".pos."\n" })
			(?(?! .* \g{cur} )
				(?{ print " cur($+{cur}) doesn't repeat ahead, looking back\n" })
				(?(?!(?&loopback))
					(?<= (?<single> . ) )
					(?{ print " cur($+{cur}) doesn't repeat behind either, "
						."FOUND SINGLE: '$+{single}'!\n" })
					(*FAIL)
				|
					(?{ print " cur($+{cur}) repeats behind\n" })
				)
			|
				(?{ print " cur($+{cur}) cur repeats ahead\n" })
			)
		)
		
		(?(?=.) # there are more characters, continue looping
			(?= (?<cur>.) (?&loopfwd) )
		)
		
		(?{ print "post: $-[0]<$&>$+[0] pos:".pos."\n" })
	) )
	(?(DEFINE) (?<loopback>
		(?<=  # this lookbehind is fixed width!
			(?<prev>  (?! \g{cur} ) . )
			(?{ print "  prev: <$+{prev}> / $-[0]<$&>$+[0] pos:".pos."\n" })
			(?=(?&loopback)) .
		|
			(?= \g{cur} ) . .
		)
	) )
	\A (?(?=(?&loopfwd))
		(?{ print "loopfwd matched (no single found)\n" })
		(*FAIL)
	|
		(?{ print "loopfwd didn't match (single found)\n" })
	)
}msx