Emacs: Regular Expression Case Sensitivity

By Xah Lee. Date: . Last updated: .

Case Sensitivity in Text Searching Commands

Case sensitivity for search is controlled by the variable case-fold-search

Variable. Configure case-sensitivity for commands that do text search, including commands that take a Emacs Regular Expression.

Value of t means smart-case. That is, if search term contain Capital letters, the search is case-sensitive, otherwise no. By default, the value is t.


  • dragon matches {dragon, Dragon, DRAGON, draGON}
  • Dragon matches only Dragon.

If nil, search is case-sensitive.

Alt+x toggle-case-fold-search
toggles the variable case-fold-search

💡 TIP: remember to toggle it back when you are done.

Case Sensitivity in Replacement, for Interactive Call

By default, the case of the replaced text is smartly dependent on the matched text.

For example, suppose your search string is here, and your replacement string is dragon. Emacs will find any of {here, Here, HERE}.

If you want the letter case of your replacement string be exactly as you have it, you need to set the variable case-replace to nil. You can do so by Alt+x set-variable.

Change Case Sensitivity in Emacs Lisp Code

In elisp code, set case-fold-search like this:

(let ((case-fold-search nil))

  (re-search-forward "ABC")

  ;; other code here

For the replacement string, the replace-match function has a option for it.

Emacs Regular Expression

Regex in Emacs Lisp