How do I evaluate expressions and insert results inline in Vim?

Explanation

The expression register (=) is one of Vim's most powerful yet underused features. While in insert mode, pressing <C-r>= opens a prompt where you can type any Vimscript expression. The result is inserted at the cursor position. This effectively turns Vim into a programmable text insertion engine.

How it works

• <C-r> — In insert mode, triggers "insert from register."
• = — Selects the expression register, opening the = prompt at the bottom of the screen.
• You type any valid Vimscript expression and press <CR> to evaluate it.
• The result is inserted as text at the cursor.

Example

Insert today's date while typing:

Before: Report created on |
(cursor at |, press <C-r>=system("date")<CR>)
After:  Report created on Thu Feb 20 10:30:00 UTC 2026

More useful expressions:

" Insert current line number
<C-r>=line(".")<CR>

" Inline math
<C-r>=2048/16<CR>         " inserts: 128

" Insert current filename
<C-r>=expand("%:t")<CR>

" Generate a numbered list item
<C-r>=line(".").'. '<CR>

Tips

• The expression register also works in command-line mode: :<C-r>= lets you compute values for Ex commands.
• Use strftime("%Y-%m-%d") for formatted dates without relying on external commands.
• You can reference any Vim variable: <C-r>=g:my_var<CR> inserts its value.
• Combine with macros: record a macro that uses <C-r>=line('.') to insert incrementing numbers.