my $re3 = qr{
	(?= (?<lookback>
		(?<=
			(?! (?<match> ab+c ) )
			(?=(?&lookback)) .
		|
			(?=(?&match)) .
		)
	) )
	(?<target> x \d+ )
}msx;
my $re3_short = qr
	/(?=((?<=(?!( ab+c ))(?=(?-2)).|(?=(?-1)).))) (x\d+) /sx;