From: Peter Constable (email@example.com)
Date: Wed Nov 09 2005 - 08:06:10 CST
Philippe's response regarding U- notation, while well-meaning, is pretty much pure fiction.
The U- notation is defined in ISO/IEC 10646. It always uses 8 hex digits, U-nnnnnnnn, and refers to a UCS-4 codepoint.
> -----Original Message-----
> From: firstname.lastname@example.org [mailto:email@example.com] On
> Behalf Of Philippe Verdy
> Sent: Tuesday, November 08, 2005 6:05 AM
> To: Dominikus Scherkl; 'Jukka K. Korpela'; firstname.lastname@example.org
> Subject: Re: Origin of the U+nnnn notation
> From: "Dominikus Scherkl" <email@example.com>
> >> I have been unable to hunt down the historical origin of the
> >> notation U+nnnn (where nnnn are hexadecimal digits) that we
> >> use to refer to characters (and code points).
> >> Presumably "U" stands for "UCS" or for "Unicode", but where
> >> does the plus sign come from?
> > Maybe it was thought of as an offset from the unit (character null)
> > like in ETA+5 minutes (expected time of arrival was passed five minutes
> > ago - an euphemism for beeing 5 minutes late).
> U-nnnn already exists (or I should say, it has existed). It was refering
> 16-bit code units, not really to characters and was a fixed-width notation
> (with 4 hexadecimal digits). The "U" meant "Unicode" (1.0 and before).
> U+[n...n]nnnn was created to avoid the confusion with the past 16-bit only
> Unicode 1.0 standard (which was not fully compatible with ISO/IEC 10646
> points). It is a variable-width notation that refers to ISO/IEC 10646 code
> points. The "U" means "UCS" or "Universal Character Set". At that time,
> UCS code point range was up to 31 bits wide.
> The U-nnnn notation is abandoned now, except for references to Unicode 1.0.
> If one uses it, it will refer to one or more 16-bit code units needed to
> encode each codepoint (possibly with surrogate pairs). It does not
> designates abstract characters or codepoints unambiguously.
> Later, the variable-width U+[n...n]nnnn notation was restricted to allow
> only codepoints in the 17 first planes of the joined ISO/IEC 10646-1 and
> Unicode standards (so the only standard codepoints are between U+0000 and
> U+10FFFF, some of them being permanently assigned to non-characters).
> The references to larger code points with U+[n...n]nnnn is discouraged, as
> they no longer designate valid code points in both standards. Their
> definition and use is then application-specific.
> There are '''no''' negative codepoints in either standards (U-0001 does
> designate the 32-bit code unit that you could store in a signed wide-char
> datatype, but in past standard it designated the same codepoint as U+0001
> now). Using "+" makes the statement about signs clear: standard code
> all have positive values.
> So if you want a representation for negative code units, you need another
> notation (for example N-0001 to represent the negative code unit with
> negative value -1): this notation is application-specific.
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