5 Lexical conventions [lex]

5.8 Header names [lex.header]

header-name:
	< h-char-sequence >
	" q-char-sequence "
h-char-sequence:
	h-char
	h-char-sequence h-char
h-char:
	any member of the source character set except new-line and >
q-char-sequence:
	q-char
	q-char-sequence q-char
q-char:
	any member of the source character set except new-line and "
[Note
:
Header name preprocessing tokens only appear within a #include preprocessing directive (see [lex.pptoken]).
end note
]
The sequences in both forms of header-names are mapped in an implementation-defined manner to headers or to external source file names as specified in [cpp.include].
The appearance of either of the characters ' or \ or of either of the character sequences /* or // in a q-char-sequence or an h-char-sequence is conditionally-supported with implementation-defined semantics, as is the appearance of the character " in an h-char-sequence.14
Thus, a sequence of characters that resembles an escape sequence might result in an error, be interpreted as the character corresponding to the escape sequence, or have a completely different meaning, depending on the implementation.