5 Lexical conventions [lex]

5.3 Characters [lex.char]

5.3.2 Universal character names [lex.universal.char]

n-char:
any member of the translation character set except the U+007d right curly bracket or new-line character
1
#
The universal-character-name construct provides a way to name any element in the translation character set using just the basic character set.
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 character set, the program is ill-formed.
[Note 1: 
A sequence of characters resembling a universal-character-name in an r-char-sequence ([lex.string]) does not form a universal-character-name.
— end note]
2
#
A universal-character-name of the form \u hex-quad, \U hex-quad hex-quad, or \u{simple-hexadecimal-digit-sequence} designates the character in the translation character set whose Unicode scalar value 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 Unicode scalar value.
3
#
A universal-character-name that is a named-universal-character designates the corresponding character in the Unicode Standard (chapter 4.8 Name) if the n-char-sequence is equal to its character name or to one of its character name aliases of type “control”, “correction”, or “alternate”; otherwise, the program is ill-formed.
[Note 2: 
These aliases are listed in the Unicode Character Database's NameAliases.txt.
None of these names or aliases have leading or trailing spaces.
— end note]