How do I force a case-insensitive substitution without changing the global ignorecase setting?

Explanation

The :s substitute command accepts /i and /I flags that override your global ignorecase and smartcase settings for that single substitution, letting you choose case sensitivity on a per-command basis.

How it works

• /i — force case-insensitive matching for this substitution, regardless of ignorecase
• /I — force case-sensitive matching for this substitution, regardless of ignorecase

This is particularly useful when you have set ignorecase or set smartcase in your vimrc but need predictable case behaviour for a specific replacement.

Example

Given the file contents:

hello Hello HELLO

With set noignorecase (the default), the following replaces all occurrences regardless of case:

:%s/hello/world/ig

Result:

world world world

Conversely, if you have set ignorecase but need an exact-case match:

:%s/Hello/World/Ig

Result (only the exact Hello is replaced):

hello World HELLO

Tips

• Combine with g for global replacement across the whole line, e.g. :%s/pat/rep/ig
• These flags also work in range substitutions: :'<,'>s/pat/rep/ig
• To make case-insensitive searching the default, add set ignorecase smartcase to your vimrc instead of using /i every time