bug#5700: emacs-23 and 8-bit characters in 128..255

[npostavs@users.sourceforge.net]

tags 5700 notabug
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With Emacs 24/25, using "\u00FF" works:

(string-equal (buffer-substring (point) (1+ (point))) "\u00FF")
(looking-at "\u00FF")

Seems to be another instance of the unibyte vs multibyte string escape syntax thing:

       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.

Stefan Monnier <monnier@IRO.UMontreal.CA> writes:
> which seems acceptable, whereas under Emacs-23 we have:
>
[...]
>   (multibyte-string-p "\377")   prints as    "\377"

In 23.4 it returns returns nil