5 Lexical conventions [lex]

5.6 Header names [lex.header]

h-char:
any member of the translation character set except new-line and U+003e greater-than sign
q-char:
any member of the translation character set except new-line and U+0022 quotation mark
[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.10
10)10)
Thus, a sequence of characters that resembles an escape sequence can result in an error, be interpreted as the character corresponding to the escape sequence, or have a completely different meaning, depending on the implementation.