my $ndn = qr{
    # cannot begin after digit or any differentiator char.
    (?<! \d) (?<! $diff)
    # begin potential main pattern capture.
    ($d_min   # begin main pattern group with minimum digits
    (?<DIFF> $diff)?  # group DIFF: possible differentiator char
    # match to max number of digit(s)/single-diff groups.
    (?: \d+ \k{DIFF} (?= \d)){0,9}
    # end main pattern group capture with minimum digits.
    $d_min)  # end main group
    # main pattern cannot be followed by a digit or...
    (?! \d)
    # ... by the diff char if any, else by any diff char.
    (?(<DIFF>) (?! \k{DIFF}) | (?! $diff))
    # qualify potential main pattern for min/max digits.
    (?(?{ $^N =~ tr/0-9// < 9 || $^N =~ tr/0-9// > 15 }) (*FAIL))
    }xms;