# Match any /\d./ that is *not* preceded by an /a/

my $re5 = qr{
	(?! (?<lookback>
		(?<=
			a
		|
			(?=(?&lookback)) .
		)
	) )
	(?<target> \d . )
}msx;
my $re5_short = qr
	/(?!((?<= a |(?=(?-1)).))) (\d.) /sx;

for my $regex ($re5,$re5_short) {
	unlike "fo",        $regex;
	unlike "x5",        $regex;
	unlike "ab5 x4",    $regex;
	  like "5ab",       $regex;
	  like "x5 ab5",    $regex;
	  like "x5 ab5 x2", $regex;
	my @results;
	while ("x2 4x3a55aaa1" =~ /$regex/g)
		{ push @results, $+{target} // $2 }
	is_deeply \@results, ["2 ","4x","3a"];
}

##</code><code>##

my $re5 = qr{
	(?<lookback>
		(?<!
			a
		|
			(?!(?&lookback)) .
		)
	)
	(?<target> \d . )
}msx;
my $re5_short = qr
	/((?<! a |(?!(?-1)).)) (\d.) /sx;