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

[David Ponce <da_vid@orange.fr>]

On 24/08/2023 12:55, David Ponce wrote:
> Hello,
> 
> While experimenting with code to create image from data, I encountered
> an issue with the regexp in `image-type-header-regexps' used to
> auto-detect PBM image type from the first bytes of image data. That is:
> 
> "\\`P[1-6]\\(?:\
> \\(?:\\(?:#[^\r\n]*[\r\n]\\)*[[:space:]]\\)+\
> \\(?:\\(?:#[^\r\n]*[\r\n]\\)*[0-9]\\)+\
> \\)\\{2\\}"
> 
> Here is a simple recipe to illustrate the issue:
> 
> In *scratch* buffer eval:
> -------------------------
> ;; Get content of a pbm file.
> (setq test-data
>        (with-current-buffer
>            (find-file-noselect "[YourEmacsPath]/etc/images/splash.pbm")
>          (prog1 (buffer-substring-no-properties (point-min) (point-max))
>            (kill-buffer (current-buffer)))))
> 
> ;; Check string data fail for pbm image-type!
> (image-type-from-data test-data)
>>>> nil
> ;; With a temp buffer current, the same test works!
> (with-temp-buffer
>   (image-type-from-data test-data))
>>>> pbm
> -------------------------
> 
> After further digging, I found that the problem might be due to the use
> of the [:space:] character class whose meaning, according to the manual,
> depends on the syntax of whitespace characters setup in current buffer.
> So, using discrete values in place of syntax class seems to solve the
> issue:
> 
> (setcar (nth 1 image-type-header-regexps)
>          "\\`P[1-6]\\(?:\
> \\(?:\\(?:#[^\r\n]*[\r\n]\\)*[ \t\r\n]\\)+\
> \\(?:\\(?:#[^\r\n]*[\r\n]\\)*[0-9]\\)+\
> \\)\\{2\\}")
> 
> (image-type-from-data test-data)
>>>> pbm
> 
> I attached a patch proposal.
> Hope it will help.
> Regards

Some additions.

Basic string matching recipe:

In *scratch* buffer eval:
-------------------------

(let ((re "\\`P[1-6]\\(?:\
\\(?:\\(?:#[^\r\n]*[\r\n]\\)*[[:space:]]\\)+\
\\(?:\\(?:#[^\r\n]*[\r\n]\\)*[0-9]\\)+\
\\)\\{2\\}")
       (text "P4
333 233"))
   (string-match-p re text))
>>> nil

(with-syntax-table (standard-syntax-table)
   (let ((re "\\`P[1-6]\\(?:\
\\(?:\\(?:#[^\r\n]*[\r\n]\\)*[[:space:]]\\)+\
\\(?:\\(?:#[^\r\n]*[\r\n]\\)*[0-9]\\)+\
\\)\\{2\\}")
         (text "P4
333 233"))
     (string-match-p re text)))
>>> 0

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?

Regards