my $re1 = qr{
	(?<target> x (?<digits> \d+ ) (?!\d) )
	(?= (?<lookback>
		(?<=
			(?! (?<match> ab \g{digits} (?!\d) ) ) .
			(?=(?&lookback)) .
		|
			(?=(?&match)) . .
		)
	) )
}msx;
my $re1_short = qr
	/ (x(\d+)(?!\d)) (?=((?<=(?!( ab\2(?!\d) )).(?=(?-2)).
		|(?=(?-1))..)))/sx;