5 Lexical conventions [lex]

5.3 Character sets [lex.charset]

1
#
The basic source character set consists of 96 characters: the space character, the control characters representing horizontal tab, vertical tab, form feed, and new-line, plus the following 91 graphical characters:11 
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
0 1 2 3 4 5 6 7 8 9
_ { } [ ] # ( ) < > % : ; . ? * + - / ^ & | ~ ! = , \ " '
2
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The universal-character-name construct provides a way to name other characters.
A universal-character-name designates the character in ISO/IEC 10646 (if any) whose code point is the hexadecimal number represented by the sequence of hexadecimal-digits in the universal-character-name.
The program is ill-formed if that number is not a code point or if it is a surrogate code point.
Noncharacter code points and reserved code points are considered to designate separate characters distinct from any ISO/IEC 10646 character.
If a universal-character-name outside the c-char-sequence, s-char-sequence, or r-char-sequence of a character-literal or string-literal (in either case, including within a user-defined-literal) corresponds to a control character or to a character in the basic source character set, the program is ill-formed.12
[Note 1:
ISO/IEC 10646 code points are integers in the range (hexadecimal).
A surrogate code point is a value in the range (hexadecimal).
A control character is a character whose code point is in either of the ranges or (hexadecimal).
— end note]
3
#
The basic execution character set and the basic execution wide-character set shall each contain all the members of the basic source character set, plus control characters representing alert, backspace, and carriage return, plus a null character (respectively, null wide character), whose value is 0.
For each basic execution character set, the values of the members shall be non-negative and distinct from one another.
In both the source and execution basic character sets, the value of each character after 0 in the above list of decimal digits shall be one greater than the value of the previous.
The execution character set and the execution wide-character set are implementation-defined supersets of the basic execution character set and the basic execution wide-character set, respectively.
The values of the members of the execution character sets and the sets of additional members are locale-specific.
11)11)
The glyphs for the members of the basic source character set are intended to identify characters from the subset of ISO/IEC 10646 which corresponds to the ASCII character set.
However, the mapping from source file characters to the source character set (described in translation phase 1) is specified as implementation-defined, and therefore implementations must document how the basic source characters are represented in source files.
12)12)
A sequence of characters resembling a universal-character-name in an r-char-sequence ([lex.string]) does not form a universal-character-name.