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

[David Ponce <da_vid@orange.fr>]

On 05/09/2023 13:08, Eli Zaretskii wrote:
> [I presume you didn't intend to discuss this only with me in private.]

Hi Eli,

You are right, my mistake, I did reply instead of reply to all :-\
I am sorry.

> 
>> 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?

Yes, according to the specification of pbm available at
<https://netpbm.sourceforge.net/doc/pbm.html>:

   "Each PBM image consists of the following:

     * A "magic number" for identifying the file type.
       A pbm image's magic number is the two characters "P4".

==> * Whitespace (blanks, TABs, CRs, LFs). <==

     * The width in pixels of the image, formatted as ASCII characters in decimal.

     ..."

Thanks