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Re: Looking at function

From: Juri Linkov
Subject: Re: Looking at function
Date: Mon, 11 Jul 2022 22:14:48 +0300
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/29.0.50 (x86_64-pc-linux-gnu)

>> 1. Replace ‘looking-at’ with a call to the search function,
>>     but keep it at point by prepending ‘\\=’ to the regexp.
>>     Can it break a complex regexp?
> I suppose it can. Even a simple one (that has \\| inside without
> a grouping).

This is what the fix for xref successfully uses in bug#53758
with changes in perform-replace from bug#14013.  (However,
none of these variants is suitable for replacing another call
of looking-at in isearch-search-and-update.)

> Do we have a clear understanding of the idea behind this looking-at call?
> The comment says:
>         ;; Otherwise, if matching a regular expression, do the next
>         ;; match now, since the replacement for this match may
>         ;; affect whether the next match is adjacent to this one.
>         ;; If that match is empty, don't use it.
> What happens if there are multiple adjacent matches in a row, not just 2?
> I suppose the replacement could be performed for the first one, then the
> next one is "popped" becoming the current and looking-at is called again
> near its end?
> If so, perhaps a good alternative is to stop caring about whether those
> matches are adjacent and always store the latest two matches, whether they
> are next to each other or not.

The sole purpose of this "do the next match now" hack
is to handle a special use case that is tested
in test/lisp/replace-tests.el:

    ;; Test case from commit 5632eb272c7
    ("a a a " "C-M-% \\ba SPC RET c RET !" "ccc") ; not "ca c"

    ;; Test case from commit 5632eb272c7
    ("a a a " "\\ba " "c" nil t nil nil nil nil nil nil nil "ccc") ; not "ca c"

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