Combining characters/Extenders, etc. in Unicode

[David Brownell]

"Paul W. Abrahams" wrote:
> 
> The XML spec defines certain character classes (in Appendix B) that are
> derived from the Unicode standard.   Could someone explain what is meant
> by Ideographics, Combining Characters, and Extenders?  I'd like to get
> this information without having to buy a whole book on Unicode, but so
> far I haven't been able to find it.

If you don't actually buy the Unicode spec, at least borrow somebody's
copy for a few days.  There's a minor education to be had even just
by skimming it ... you'll be able to tell what this summary gets wrong,
for example!  ;-)

Ideographics ... think of Chinese characters.  There's a big problem
with Unicode 2.x in that it doesn't handle enough of them; there are
many more coming in some eventual revision of Unicode (perhaps in
the surrogate blocks).  (Leading to an interesting question of whether
those characters will be usable in names in some revision of XML.)

Combining characters ... think of accented characters.  You can write
them sometimes as one character, sometimes as two, sometimes as both.
(So there's a normalization question -- when you compare, you need to
have a consistent representation of said combined characters).  It gets
much more interesting for non-western languages ... in general, as with
surrogate pairs, characters don't consist of single sixteen bit Unicode
values even though that'd make algorithms simpler!

Extenders ... you'd have to look at the Unicode spec to see.  Looks
like a lot of language-specific issues come up.

- Dave

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