How do I use the expression register in a mapping to insert dynamic values?

Explanation

The expression register (=) evaluates Vimscript expressions and uses the result as register content. When combined with mappings, it creates powerful shortcuts that insert dynamic values like dates, calculations, or formatted text with a single keystroke.

How it works

• "= — access the expression register
• expression<CR> — evaluate the expression
• p — paste the result
• In a mapping, the entire sequence runs with one keypress

Example

" Insert today's date
nnoremap <leader>d "=strftime('%Y-%m-%d')<CR>p

" Insert a UUID-like string
nnoremap <leader>u "=system('uuidgen')[:-2]<CR>p

" Insert current line number as text
nnoremap <leader>l "=line('.')<CR>p

Press <leader>d:
2024-01-15 (inserted at cursor)

Press <leader>l:
42 (current line number inserted)

Tips

• Use P instead of p to insert before the cursor
• In insert mode, use <C-r>=expression<CR> for inline insertion
• system() runs shell commands — trim trailing newline with [:-2]
• strftime() supports all standard format specifiers: %H:%M, %A, etc.