C99 supports universal character names that may be used in identifiers, character constants, and string literals to designate characters that are not in the basic character set. The universal character name \Unnnnnnnn designates the character whose eight-digit short identifier (as specified by ISO/IEC 10646) is nnnnnnnn. Similarly, the universal character name \unnnn designates the character whose four-digit short identifier is nnnn (and whose eight-digit short identifier is 0000nnnn).
C99, Section 5.1.1.2, paragraph 4, says:
If a character sequence that matches the syntax of a universal character name is produced by token concatenation (6.10.3.3), the behavior is undefined.
In general, universal character names should be avoided in identifiers unless absolutely necessary. The basic character set should suffice for almost every identifier.
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