Case insensitive search with M-x find-grep-dired

Question

I’m trying to do a case insensitive search with M-x find-grep-dired. When I enter M-x find-grep-dired and hit ENTER and enter the directory name I get the prompt find-grep (grep regexp):. I understand this as “enter grep and then regexp”. So, for grep I enter -i for case insensitive search, and for regexp I enter the search word. But this does not work. What am I doing wrong?

Answer 1

The second prompt is asking you for just a "grep regexp", i.e. a regexp conforming to grep syntax [1]. If you look at the implementation of find-grep-dired (C-h f find-grep-dired and click on the file link), you'll see that it uses find-grep-options, so you can add -i to the value of that variable. You can do that by asking for help on the variable (C-h v find-grep-options) and then clicking on the Customize link.

EDIT: if you don't want to change the value of find-grep-dired permanently, you can use the standard lisp facility of binding a variable temporarily with let:

(let ((find-grep-options "-q -i"))
   (call-interactively #'find-grep-dired))

That's a very general mechanism that is worth remembering and getting familiar with.

And you can of course package that in a command and bind it to a key if you want:

(defun find-grep-dired-case-insensitive ()
   (interactive)
   (let ((find-grep-options "-q -i"))
      (call-interactively #'find-grep-dired)))
(define-key global-map (kbd "C-c z") #'find-grep-dired-case-insensitive)

Footnote:

[1] There are other regexp syntaxes: e.g. perl regexp syntax is different in many ways from that of (basic) grep; GNU grep can use an extended syntax if you give it a -E option; Emacs has its own syntax for regexps. They are all similar in many ways, but differ in others. That's why the documentation of the function says to use a "grep regexp".