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my ($after)  = $str =~ /^.{$pos}(\d*)/;
my ($before) = length $after
               ? reverse($str) =~ /(?=(\d*))(?=.{$pos}$)/
               : "";
my $match    = reverse($before) . $after;

    $str =~ /^.{$pos}
             ( (?<= (?= \D | ^ | (?1) (\d\d) ) ) )
             (\d+)
            /x;