5 Lexical conventions [lex]

5.8 Header names [lex.header]

h-char:
any member of the source character set except new-line and >
q-char:
any member of the source character set except new-line and "
[Note 1:
Header name preprocessing tokens only appear within a #include preprocessing directive, a __has_­include preprocessing expression, or after certain occurrences of an import token (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.16
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.