> The following form evals to nil:
>
> (string-equal "\377" "ÿ")
"\377" is a unibyte string. When converted to multibyte it yields
"\x3fffff".
At least as of 24.3, the manual[0] suggests that such a conversion should not occur in this case:
You can also use hexadecimal escape sequences (`\xN') and octal
escape sequences (`\N') in string constants. *But beware:* If a string constant contains hexadecimal or octal escape sequences, and these escape sequences all specify unibyte characters (i.e., less than 256), and there are no other literal non-ASCII
characters or Unicode-style escape sequences in the string, then Emacs automatically assumes that it is a unibyte string. That is to say, it assumes that all non-ASCII characters occurring in the string are 8-bit raw bytes.