Is the backslash acceptable in C and C++ #include directives?

Question

There are two path separators in common use: the Unix forward-slash and the DOS backslash. _{^{Rest in peace, Classic Mac colon.}} If used in an #include dir

Answer 1

The standard says for #include that it:

searches a sequence of implementation-defined places for a header identified uniquely by the specified sequence between the delimiters, and causes the replacement of that directive by the entire contents of the header. How the places are specified or the header identified is implementation-defined.

Note the last sentence.