Character Range: surrogate blocks
cowan at locke.ccil.org
Sun Oct 18 00:42:10 BST 1998
Richard Emberson scripsit:
> To extend the available characters in Unicode one
> can use to 16 bit characters with surrogate blocks.
Well, no. One uses the 16-bit *codes* in the surrogate blocks.
They aren't *characters* (what in Unicode is called *abstract characters*)
> Now in production rule #2 titled Character Range
> surrogate blocks are explicitly excluded (along
> with FFFF and FFFE).
Correct, because these codes do not represent characters.
> Does that mean that if one were reading a character
> stream that included characters not in the basic
> set of Unicode characters (those not using surrogate
> blocks) that it would be a wellformedness violation?
I don't understand this question. Is there an extra "not" somewhere?
> There are the extra, beyond 16-bit, characters specified
> by the spec in production rule #2 as "[x10000-#x10FFFF]".
> Is this how Unicode characters that use the surrogate
> blocks get represented in an XML document?
In UTF-16 (= Unicode) representation, yes. In UTF-8 representation
they are represented as the appropriate 4-byte sequences.
> Is there
> an algorithm for the convertions defined somewhere?
Same as Unicode or ISO 10646 UTF-16, namely: a codepoint in the
range D800-DBFF followed by one in the range DC00-DFFF represents
the character whose code is (first-D800)*400+(second-DC00)+10000
> Short of getting a copy of the Unicode 2.0 spec, is there
> anywhere where the conversion algorithm is documented?
> Why was it decided to exclude the uses of surrogate
> block-base Unicode characters within XML documents?
What is excluded is surrogate characters appearing in unpaired form.
These could be generated, e.g. by UTF-8 ED A0 80 = U+D800.
John Cowan cowan at ccil.org
e'osai ko sarji la lojban.
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