Pasting modified register contents into Visual Block selection

Question

Consider this mapping:

vnoremap <leader>U ygv"=toupper(@@)<cr>p

It will yank the current visual selection, select it back, and then put a modified version of it. In this case, uppercase letters.

This works great on character-wise and linewise Visual modes, where the final p command will replace the contents of the previous selection, leaving everything else in place.

However, in block-wise Visual mode, it doesn't work as expected. Even though the resulting register has the same "format" as the one before the transformation, the put operation ends up shifting things around, putting the multi-line content in the first line of the block and breaking some lines as it does that.

This same problem doesn't happen if I simply put the register, unmodified. Inspecting the register with :reg " doesn't show anything out of the ordinary.

In fact, applying the transformation manually with :let @@ = toupper(@@) and inspecting the register afterwards doesn't show anything that's been lost, but putting it into a Visual block selection still fails at that point.

So I ask:

• Is there some magic in how contents coming from Visual block yanks/deletes are stored into registers?
• If so, is there anyway to inspect that, see it?
• Is there any way to reproduce that, so that after the toupper() conversion I could still be able to preserve that it's meant to be a block?
• If that's not possible, are there any workarounds available for this situation?

I looked at help for Visual block mode and didn't find anything (in fact, didn't find specific help on v_b_p.)

Also, toupper() is just a simple example. Of course I could simply use U for that case. The use of toupper() is just as a proxy for a more general transformation. (This issue came up as I was writing an answer to this question.)

Answer 1

To modify the content of a register while keeping its type (characterwise, linewise or block) you need to use setreg(). Otherwise Vim is only able to choose the register type as characterwise or linewise. There is note at :help setreg() which hints at this:

Note: you may not reliably restore register value without using the third argument to getreg() as without it newlines are represented as newlines AND Nul bytes are represented as newlines as well, see NL-used-for-Nul.

In your example I would use the following command to uppercase the blockwise selection in the default register:

call setreg('"', toupper(@"), getregtype('"'))

(I chose to use the default register instead of @@, because in my tests getregtype('@@') returned an empty string.)