be able to replace during isearch

[Drew Adams]

I have this feature in Icicles search, but the idea is not dependent on
Icicles. I think that a feature like this would also be useful to add to
isearch.

The idea is that you would not need to leave isearch to perform
replacements. Whenever you wanted to replace the current search occurrence,
you would hit a certain key. The first time you do this, you would be asked
for the replacement string.

The main advantage, besides not needing to exit isearch, is that instead of
being queried for each potential replacement, one by one, you replace only
those you want upon demand. You might not even have the intention of
performing replacement until you happened to notice, while searching, that
you wanted to change something. A minor advantage is that you can also
search-and-replace backward.

Nice-to-haves: In Icicles search, you can use any replacement expression
that is accepted by `query-replace-regexp', and there are additional key
bindings during search to (1) replace all and (2) redefine the replacement
to use. These would also be useful additions for isearch.

I'm not suggesting this as a replacement(!) for query-replace, but as an
enhancement to isearch. Query-replace is a different UI, and each has its
advantages.

Here is a bit more explanation of this feature in Icicles, but, as I say,
the idea is not Icicles-specific:
http://www.emacswiki.org/cgi-bin/wiki/Icicles_-_Search-And-Replace.