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

my $re2 = qr{
    (?<char> . )
...
}msx;
my $re2_short = qr
    /(.)(?=((?<=(?!\1).(?=(?-1)).|(?=\1)..)))/sx;

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

my $re4 = qr{
    (?= (?<lookback>
...
}msx;
my $re4_short = qr
    /(?=((?<= a |(?=(?-1)).))) (\d.) /sx;

#!/usr/bin/env perl
use warnings;
...
}

done_testing;