Looking at function

[Juri Linkov <juri@linkov.net>]

Any use of the function ‘looking-at’ is incompatible with a non-default value
of ‘isearch-search-fun-function’.  So there are following problematic uses
of ‘looking-at’:

1. There is 1 call in isearch.el in ‘isearch-search-and-update’:

		   (looking-at (cond
				((functionp isearch-regexp-function)
				 (funcall isearch-regexp-function isearch-string t))
				(isearch-regexp-function (word-search-regexp isearch-string t))
				(isearch-regexp isearch-string)
				(t (regexp-quote isearch-string))))

   It doesn't call a non-default search function when using a key sequence like
   ‘C-M-r ^’ on rectangular regions — it matches outside of regions
   when the search function restricts the search within the region bounds.

2. In replace.el ‘looking-at/back’ are used only in ‘perform-replace’
   to check if the next match is adjacent.  This causes problems during
   rectangular replacements to find matches based on a non-default
   search function.

These possible solutions were proposed in bug#54733:

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?

2. Call the search function as above but afterwards check if
   (match-beginning 0) is equal to the original position.
   Less efficient.

3. Use looking-at only when the search function is default.

4. Add a new variable ‘looking-at-function’.
   It could be like ‘isearch-search-fun-function’, so redefining
   the search function will also require redefining the
   looking-at function with similar code that doesn't move point.

Any better ideas how to replace looking-at with something
that uses a search function?