bug#49588: 28.0.50; (number-at-point) doesn't work with hex prefixes

[Remington Furman]

This is related to bug#37458 (which built on bug#8634).

(number-at-point) returns 0 when the point is at the beginning of
"0x100" and returns 100 (decimal) when the point is at the end of "0x100".  I
expect it to return 256 in either location.

(number-at-point) works as expected and returns 256 when the point is at
the "#" of "#x100".  It returns nil when the point is on "x", and 100
elsewhere.

The following works as expected (though I don't suggest it's the best
possible implementation):

(defun number-at-point ()
  "Return the number at point, or nil if none is found.
Decimal numbers like \"14\" or \"-14.5\", as well as hex numbers
like \"0xBEEF09\" or \"#xBEEF09\", are recognized."
  (cond ((thing-at-point-looking-at "\\(0x\\|#x\\)\\([a-fA-F0-9]+\\)" 500)
         (string-to-number
          (buffer-substring (match-beginning 2) (match-end 2))
          16))
        ((thing-at-point-looking-at "-?[0-9]+\\.?[0-9]*" 500)
         (string-to-number
          (buffer-substring (match-beginning 0) (match-end 0))))))

It requires two calls to (thing-at-point-looking-at) for the common case
of decimal numbers, which is unfortunate.

I also tried re-ordering the capture groups around the alternation in
the regular expression to look for hexadecimal numbers first, but that
doesn't fully work.  The following version works with the point anywhere
in "0x100", but still only works when the point is at the beginning of
"#x100".

(defun number-at-point ()
  "Return the number at point, or nil if none is found.
Decimal numbers like \"14\" or \"-14.5\", as well as hex numbers
like \"0xBEEF09\" or \"#xBEEF09\", are recognized."
  (when (thing-at-point-looking-at
         "\\(0x\\|#x\\)\\([a-fA-F0-9]+\\)\\|\\(-?[0-9]+\\.?[0-9]*\\)" 500)
    (if (match-beginning 1)
        (string-to-number
         (buffer-substring (match-beginning 2) (match-end 2))
         16)
      (string-to-number
       (buffer-substring (match-beginning 3) (match-end 3))))))

I haven't sorted out exactly why the alternation group confuses
(thing-at-point-looking-at), but I think it's because it searches
forward and backwards so the decimal group will match a portion of a
hexadecimal number.

In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.20, cairo 
version 1.16.0)
 of 2021-07-13 built on ubuntu
Repository revision: dd34bef7d3769a8574bcee2c1e91e8445129af75
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12009000
System Description: Ubuntu 20.04.2 LTS

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Major mode: Lisp Interaction

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Load-path shadows:
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Memory information:
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