How do I do a case-preserving search and replace in Vim?

Explanation

Standard substitutions don't preserve the original case of matched text. Using \= expression replacement with submatch() and case functions, you can replace words while keeping their original capitalization pattern — uppercase stays uppercase, lowercase stays lowercase.

How it works

• \= in the replacement enables expression evaluation
• submatch(0) returns the full matched text
• toupper() / tolower() convert case
• By checking the case of the original, you can mirror it in the replacement

Example

Replace "old" with "new" preserving case:

:%s/\v\c(old)/\=submatch(0)[0]=~#'[A-Z]' ? 'New' : 'new'/g

Before: Old and old and OLD
After:  New and new and NEW (with additional logic for all-caps)

Tips

• The abolish.vim plugin provides :S/old/new/g (Subvert) for easy case-preserving substitution
• For simple two-case scenarios, run two substitutions: :%s/Old/New/g | %s/old/new/g
• Use \C to force case-sensitive matching: :%s/\Cold/new/g
• submatch(0) =~# '[A-Z]' tests if the match starts with uppercase