How do I evaluate a Vimscript expression and insert the result into a command-line command?

Explanation

Just like <C-r>= lets you insert evaluated expressions in insert mode, you can use it inside an Ex command on the command line to embed any Vimscript expression result mid-command. This lets you build dynamic commands — computing filenames, line numbers, timestamps, or buffer values — without leaving the command line or writing a separate mapping.

How it works

1. Start typing an Ex command (e.g., :%s/old/)
2. At the point where you want a dynamic value, press <C-r>=
3. The = prompt appears; type any Vimscript expression and press <CR>
4. The evaluated result is inserted into the command as a plain string
5. Finish the command normally

Example

Replace TODO with today's date:

:%s/TODO/<C-r>=strftime("%Y-%m-%d")<CR>/g

After pressing <CR> on the expression, it becomes:

:%s/TODO/2026-03-07/g

Open a file path computed from the word under cursor:

:e <C-r>=expand('<cfile>')<CR>

Tips

• Any Vimscript function works: line('.'), col('.'), shellescape(expand('%')), system('hostname')
• Works in search patterns too: press / then <C-r>= to embed a computed regex
• Combine with <C-r><C-w> (word under cursor) and <C-r><C-a> (WORD under cursor) for other inline insertions
• See :help c_CTRL-R_= for full documentation