Siehe
http://www.open-std.org/jtc1/sc22/wg...onaleV5.10.pdf ...
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A new feature of C99: C99 adds the concept of universal character name (UCN) (see §6.4.3) in
order to allow the use of any character in a C source, not just English characters. The primary
goal of the Committee was to enable the use of any “native” character in identifiers, string
literals and character constants, while retaining the portability objective of C.
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6.4.3 Universal character names
A new feature of C99: Note that, to allow for Universal Character Names (UCNs), a new
production has been added to the grammar that encompasses all forms of identifier elements
(basic letter, UCN, or extended character). There was some discussion about the need to require
5 an implementation to
handle all digits, Arabic or otherwise, in a similar way. The general
feeling was that detecting the “extended digits” might be an undesirable burden for many
implementations and should be avoided if possible.
Note that a strictly conforming program may use in identifiers only the extended characters listed
in Annex I, and may not begin an identifier with an extended digit.
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QUIET CHANGE IN C99
Character literals of the form ‘\unnnn’ and ‘\Unnnnnnnn’ now have different
meanings (see §6.4.3). Note that the escape sequence beginning with \U is reserved
in C99, but was not reserved in C89.