identifier-nondigit: nondigit universal-character-name other implementation-defined characters
nondigit: one of a b c d e f g h i j k l m n o p q r s t u v w x y z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z _
digit: one of 0 1 2 3 4 5 6 7 8 9
An identifier is an arbitrarily long sequence of letters and digits. Each universal-character-name in an identifier shall designate a character whose encoding in ISO 10646 falls into one of the ranges specified in [charname.allowed]. The initial element shall not be a universal-character-name designating a character whose encoding falls into one of the ranges specified in [charname.disallowed]. Upper- and lower-case letters are different. All characters are significant.20
The identifiers in Table [tab:identifiers.special] have a special meaning when appearing in a certain context. When referred to in the grammar, these identifiers are used explicitly rather than using the identifier grammar production. any ambiguity as to whether a given identifier has a special meaning is resolved to interpret the token as a regular identifier.
In addition, some identifiers are reserved for use by C++ implementations and standard libraries ([global.names]) and shall not be used otherwise; no diagnostic is required.
On systems in which linkers cannot accept extended characters, an encoding of the universal-character-name may be used in forming valid external identifiers. For example, some otherwise unused character or sequence of characters may be used to encode the \u in a universal-character-name. Extended characters may produce a long external identifier, but C++ does not place a translation limit on significant characters for external identifiers. In C++, upper- and lower-case letters are considered different for all identifiers, including external identifiers.