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

...
        { push @results, $+{target} // $2 }
    is_deeply \@results, ["2 ","4x","3a"];
}

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