Combining mark vs combining character?

Simon Josefsson simon at josefsson.org
Wed Jan 5 15:07:43 CET 2011


Vint Cerf <vint at google.com> writes:

> Simon,
>
> I am pretty sure that the terms "combining mark" and "combining character"
> as used in IDNA2008 mean the same thing.
>
> neither are permitted as the initial character of a Unicode domain label

Thanks.  And the practical definition of what a combining mark&character
is that it has a General Category of M as explained in section 3.6 of
Unicode 5.0 quoted below?

Note that this is different than having a non-0 Combining Class value.

/Simon

> vint
>
>
>
>
> On Wed, Jan 5, 2011 at 5:06 AM, Simon Josefsson <simon at josefsson.org> wrote:
>
>> Hi,
>>
>> I need a clarification regarding this paragraph in section 4.2.3.2 of
>> RFC 5891:
>>
>>   The Unicode string MUST NOT begin with a combining mark or combining
>>   character (see The Unicode Standard, Section 2.11 [Unicode] for an
>>   exact definition).
>>
>> And this in section 5.4:
>>
>>   Putative U-labels with any of the following characteristics MUST be
>>   rejected prior to DNS lookup:
>> ...
>>   o  Labels whose first character is a combining mark (see The Unicode
>>      Standard, Section 2.11 [Unicode]).
>>
>> The reference to [Unicode] is not normative, which would be a problem
>> for any implementer.
>>
>> Reading section 2.11 of Unicode 5.0 discuss "combining character" but
>> not "combining mark".
>>
>> There is a section 7.9 in Unicode 5.0 called "Combining Marks".
>>
>> A section that discuss both Combining Marks and Combining Characters in
>> the same section is section 3.11 on "Canonical Ordering Behaviour".
>>
>> There is one section 3.6 on "Combination" that gives the precice
>> definition of a "Combining character":
>>
>>   Combining character: A character with the General Category of
>>   Combining Mark (M).
>>
>> Is this the intended definition of Combining character by RFC 5891?
>>
>> Questions:
>>
>> 1) Does RFC 5891 refer to "combining mark" and "combining character" as
>> the same thing?
>>
>> 2) Is there a significant difference between the requirement in 4.2.3.2
>> and 5.4?  The latter section only mentions "combining mark" and not
>> "combining character".
>>
>> 3) What is the precice definition of a "combining mark"?
>>
>> /Simon
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