2 Lexical conventions [lex]

2.9 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 "

Header name preprocessing tokens shall only appear within a #include preprocessing directive ([cpp.include]). 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.19

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.