bug#65496: 30.0.50; Issue with the regexp used to auto-detect PBM image data

[Eli Zaretskii <eliz@gnu.org>]

[I presume you didn't intend to discuss this only with me in private.]

> Date: Mon, 4 Sep 2023 23:43:56 +0200
> From: David Ponce <da_vid@orange.fr>
> 
> On 04/09/2023 19:36, Eli Zaretskii wrote:
> >> Date: Mon, 4 Sep 2023 18:32:22 +0200
> >> From: David Ponce <da_vid@orange.fr>
> >>
> >> I wonder if it is expected that matching a regular expression
> >> against a string object depends on the syntax-table setup in current
> >> buffer?  Shouldn't (standard-syntax-table) implied when matching a
> >> regexp against a string object, that is, regardless of any buffer
> >> context?
> > 
> > Not necessarily, because you wouldn't expect, say, looking-at to
> > return a different result than (string-match-p (buffer-string)), would
> > you?
> 
> Sure, from this perspective you are right.  However, for other cases
> where the string object is not related to a buffer value, it's not so
> clear ;-)
> 
> > This belongs to the gray areas of Emacs.  The same situation exists
> > with functions like downcase, which use the buffer-local value of
> > case-table.
> 
> I can understand that.  Many things are not only black or white ;-)
> 
> Maybe for the use case of auto-detecting image type from image data,
> my proposed patch to replace character class by a list of unambiguous
> explicit character values in the regexp could make sense?

Yes, it makes sense, but are you sure you mention there all the
characters that can happen in PBM images, and only those characters?