How do I capitalize the first letter of every word in a file using a substitute command?

Explanation

Vim's substitute command supports case-conversion modifiers in the replacement string. The \u modifier capitalizes the next character, and & represents the entire matched text. Combining them as \u& capitalizes just the first letter of each match, turning a file into title case in a single command.

How it works

• % — apply to all lines in the file
• s/ — substitute command
• \w\+ — match one or more word characters (a whole word)
• /\u&/g — replace with the match (&) with its first character uppercased (\u)

The key insight: & in the replacement refers to the entire match, and \u applies only to the first character of whatever follows it.

Example

Before:

the quick brown fox jumps over the lazy dog

After :%s/\w\+/\u&/g:

The Quick Brown Fox Jumps Over The Lazy Dog

Tips

• Use \U& instead of \u& to uppercase the entire word, not just the first letter
• For a selected range: :'<,'>s/\w\+/\u&/g
• To lowercase every word instead: :%s/\w\+/\L&/g
• The \u, \l, \U, \L modifiers work in any :substitute replacement — not just with &
• \E or \e ends a \U or \L case region mid-replacement