Re: bug#31636: 27.0.50; lockfile syntax searchable from info manual

[Noam Postavsky <npostavs@gmail.com>]

On 6 June 2018 at 03:43, Robert Pluim <rpluim@gmail.com> wrote:

>> And I think you'll want to give a limit
>> to the looking-back calls, otherwise they can keep searching until the
>> beginning of buffer.
>
> ((looking-back "(") should never look at more than one character.

Perhaps it shouldn't, but Emacs doesn't make that kind of optimization:

(with-temp-buffer
  (let ((last-command-event ?a))
    (dotimes (i 8)
      (self-insert-command (expt 10 i))
      (benchmark 1 '(looking-back "(")))))
Elapsed time: 0.000036s
Elapsed time: 0.000030s
Elapsed time: 0.000066s
Elapsed time: 0.000427s
Elapsed time: 0.004068s
Elapsed time: 0.040317s
Elapsed time: 0.422173s
Elapsed time: 1.514244s

(with-temp-buffer
  (let ((last-command-event ?a))
    (dotimes (i 8)
      (self-insert-command (expt 10 i))
      (benchmark 1 '(looking-back "(" (1- (point)))))))
Elapsed time: 0.000031s
Elapsed time: 0.000027s [5 times]
Elapsed time: 0.000041s
Elapsed time: 0.000045s

> I could always do (eq (char-before) ?() I suppose.

That's probably better. But wait! This time you *should* have a backslash: ?\(

> The regexp produced by (sentence-end) looks like itʼs fairly well
> anchored. Besides, Iʼm not searching for an end-of-sentence, Iʼm
> asking "Am I at end-of-sentence".

Which is exactly why you should put a limit.