my $ndn = qr{
    # cannot begin after digit or any differentiator char
    (?<! \d) (?<! $diff)
    # begin potential main pattern capture to group 1
    ($d_min   # begin group 1 with minimum digits
    ($diff)?  # group 2: possible differentiator char
    # match to max number of digit(s)/single-diff groups
    (?: \d+ \g{-1} (?= \d)){0,9}
    # end group 1 (main pattern) capture with minimum digits
    $d_min)  # end group 1
    # main pattern cannot be followed by a digit...
    (?! \d)
    # ...or by the diff char, or by any diff char if none present
    (?(2) (?! \g{-1}) | (?! $diff))
    # qualify potential main pattern for min/max digits
    (?(?{ $1 =~ tr/0-9// > 15 || $1 =~ tr/0-9// < 9 }) (*FAIL))
    }xms;