How do I replace the Vim command-line content with the result of a Vimscript expression?

Explanation

Pressing <C-\>e while in command-line mode (:, /, ?, or !) evaluates a Vimscript expression and replaces the entire command-line with its result. This lets you build commands dynamically using Vim functions without leaving the command line.

How it works

1. Start typing any command on the : (or /) prompt
2. Press <C-\>e — the cursor moves to a mini expression prompt
3. Type a Vimscript expression and press <CR>
4. The result replaces whatever was on the command line
5. Press <CR> again to execute the resulting command

Example

Suppose you want to open a file in the same directory as the current file. Instead of typing the path manually:

:e <C-\>e 'e ' . expand('%:p:h') . '/'<CR>

This evaluates 'e ' . expand('%:p:h') . '/' and replaces the command line with something like :e /home/user/myproject/, ready for Tab completion.

Another example — substitute using a pattern from a register:

:%s/<C-\>e @a<CR>//g

This replaces the pattern field with the contents of register a.

Tips

• Works in all command-line modes: :, /, ?, and !
• Combine with expand(), getcwd(), getline('.'), and other Vimscript functions
• To insert just a function result into the command line (not replace it), use <C-r>= instead