bug#12054: 24.1; regression? font-lock no-break-space with nil nobreak-char-display

["Drew Adams" <drew.adams@oracle.com>]

> > Just why is it that the regexp "[\240]+" does not match this char?
> 
> Because, for histerical reasons, 'insert' treats strings such as
> "\nnn" as unibyte strings.

Sorry, I don't understand your point.  My question was about the regexp (not)
matching, not about (not) being able to insert the char.

I don't see a problem with inserting the char.  As I said, the correct char gets
inserted AFAICT, as shown both by `C-u C-x =' and by Yidong's correction of the
font-lock regexp.

You can insert the _same_ char using either `C-q 240' or `C-x 8 RET no-break
space', at least AFAICT (via Yidong's highlighting and via `C-u C-x =').

> > Why should a character-alternative expression care whether the
> > representation is unibyte or multibyte?  Isn't that a bug?
> 
> It's an unfortunate dark corner, due to the ambiguity of what \240
> really means in a string.

That just makes it darker for me.  Can you please elaborate?

> > How to use octal syntax to match that char?
> 
> Why do you need the octal syntax?  Why not just use a literal  ?  Is
> that only for the sake of old Emacs versions, or for some other
> reason?

1. Yes, for the sake of older Emacs versions.

2. The manual says that octal syntax is the most general syntax.
So one would expect that one can use it more, not less. ;-)

3. Why not?  Why turn it around and speak of "need" to use it?
The real question is why _not_ be able to use octal syntax here?

> > The Elisp manual says clearly that
> > "The most general read syntax for a character represents 
> > the character code in either octal or hex."
> >
> > MOST GENERAL, not most limited and partial.
> 
> I see no contradiction or incorrect information in this cited text.
> The octal notation does work in your example, it's just that its
> semantics is not what you expected.  Or am I missing something?

Dunno whether you are missing something.  I am missing how the octal notation
"works" in my example.  It certainly does not highlight the char I want to
highlight, i.e., does not do what I intended.  How to do that?

I'm missing how to use octal notation in such a font-lock-add-keywords sexp to
match that char.  IOW, my incorrect use of it doesn't do the job.  Please show
me how to use octal notation to get that char highlighted.