How to make insert mode mapping count as multiple undos?

Question

I have the following mappings:

nnoremap # /___<CR>ciw
imap # <Esc>:w<CR><Esc>#

And the following file that I am editing:

foo ___
goo ___
hoo ___

The idea is that I start at the beginning of the file, type # and I am editing the first ___, then while still in insert mode, I type # again, and I am editing the next ___, saving between each edit. This works fine, but the problem is that if I make a mistake, and type u, it undoes ALL the edits and reverts ALL of them back to ___ instead of just the most recent change.

How can I do these mappings in a way so that each time I save, it counts as a separate undo?

Answer 1

You can use <c-g>u in insert mode to break undo sequence and start new change. Like this:

imap # <c-g>u<Esc>:w<CR>#

(removed the second <esc>, as it is not needed.)

See :help i_CTRL-G_u

Answer 2

From :h undojoin:

One undo command normally undoes a typed command, no matter how many changes
that command makes.  This sequence of undo-able changes forms an undo block.
[...]
To do the opposite, break a change into two undo blocks, in Insert mode use
CTRL-G u.  This is useful if you want an insert command to be undoable in
parts.  E.g., for each sentence.  i_CTRL-G_u
Setting the value of 'undolevels' also breaks undo.  Even when the new value
is equal to the old value.

So, you could modify your second mapping to break the undo block (before the write):

imap # <C-G>u<Esc>:w<CR><Esc>#

Or the fist mapping to start a new block with input:

nnoremap # /___<CR>ciw<c-g>u