Here is a further simplified (and tested) version of the regex. The  $digits and  $diffs package variables are no longer needed, so I'm a little happier with this version, but it still uses absolute capture group numbering and embedded code. I could perhaps use named captures to get around the numbering problem, but I don't see what I can do about the code.

There are a few more comments that may be helpful, and davido's nice Perl Regular Expression Tester may be enlightening. I may get around to posting a more detailed commentary on the regex in the next couple of days. 

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;

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In reply to Re^7: Regex Modification by AnomalousMonk
in thread Regex Modification by Anonymous Monk

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