Autocomplete incremental search behaviour

Question

Consider the following scenario

Content of the buffer:

I saw the water in a thermal bath.

Commands issued:

:set incsearch

/th^R^W^R^W^R^W^R^W

Result

thewaterIII

Question

What exactly is the algorithm of the incremental search autocomplete in this case? Why does it behave in such a strange way?

In particular:

• why does it add water without a space?
• why does it add only the first I letter of the next word in?
• why is that letter capitalized?
• why does it get stuck there and does not proceed further?

I would have expected something like this:

^R^W the

^R^W^R^W the water

^R^W^R^W^R^W the water in

... etc

Answer 1

why does it add water

C-r C-w inserts the word positioned under your cursor; however, if 'incsearch' is set, and there is a match in the buffer for the current contents of the command-line, Vim doesn't use the cursor position anymore, but the end of the match. From :h c^r^w:

When 'incsearch' is set the cursor position at the end of the
currently displayed match is used.

When your command-line contains the, the end of the match is here:

I saw the water in a thermal bath.
         ^

There is no word at that position, so Vim inserts the next word which is water. I guess Vim uses a similar algorithm as with the * command:

*                       Search forward for the [count]'th occurrence of the
                        word nearest to the cursor.  The word used for the
                        search is the first of:
                                1. the keyword under the cursor |'iskeyword'|
                                2. the first keyword after the cursor, in the
                                   current line
                                3. the non-blank word under the cursor
                                4. the first non-blank word after the cursor,
                                   in the current line
                        Only whole keywords are searched for, like with the
                        command "/\<keyword\>".  |exclusive|
                        'ignorecase' is used, 'smartcase' is not.

without a space?

I guess that's because C-r C-w is meant to insert a word, and a space character is not considered as being part of a word.

why does it add only the first I letter of the next word in?

It's not the first letter of the next word; it's the word under your cursor.

I saw the water in a thermal bath.
^

When your search command-line contains thewater, Vim can't use the end of the match as the position of the word because there is no match for thewater in your buffer; so it falls back to your cursor position. And under your cursor, the word is I, not in.

why is that letter capitalized?

Again, it's not just a capitalized letter; it's the word under your cursor.

why does it get stuck there and does not proceed further?

Because no matter how many I you insert, there is still no match in the buffer; so Vim keeps using the cursor position to decide which word to insert.

---

To insert multiple words from the current line, you could also:

1. press C-r C-l to insert the whole contents of the line (see :h c^r^l)
2. use traditional editing commands to remove the words you're not interested in (see :h ex-edit-index)

Note that c^r^l requires the patch 8.0.1787. Without, you could try some custom mapping (e.g. cno <c-r><c-l> <c-r><c-r>=getline('.')<cr>).

Answer 2

user938271's great answer already answers your explicit questions about what happens when you press Ctrl-RCtrl-W during an incremental search.

I just wanted to suggest that you might want to try pressing Ctrl-L instead. During a search with 'incsearch' set, this adds the next character after the current match each time it is pressed.

So with your example, typing:

/th^L^L^L^L^L^L^L

Will result in:

/the water

(You can of course hold down the keys to auto-repeat.)

See :help /_CTRL-L for more details.