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Re: Looking at function

From: Dmitry Gutov
Subject: Re: Looking at function
Date: Tue, 12 Jul 2022 03:19:42 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.9.1

On 11.07.2022 22:14, Juri Linkov wrote:
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.)

Right. Because xref basically uses literal matching, no alternations.

But it will break more complex cases.

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"

All right. So it seems the idea of keeping references to the two latest can work.

No idea how big the changes will have to be, though.

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