"replace-match" leaves first letter capitalized

Question

I'm writing a little cleanup function that will search for the word 'let' in a case insensitive manner then replace it with the lowercase 'let'. It works in all my test cases, except for when the first letter is capitalized:

leT x=1
Let y=2
let z=3
lEt a=3

In the output, the second line is wrong:

let x=1
Let y=2
let z=3
let a=3

Here's the elisp:

(defun let-cmd()
  (goto-char (point-min))
  (while (search-forward-regexp "Let" nil t) 
    (replace-match "let" (match-string 1)) t nil))

(defun cleanup()
  (interactive)
  (let-cmd))

How do I fix this one example?

Answer 1

You can pass a non-nil value as the FIXEDCASE parameter. From the built-in help:

(replace-match NEWTEXT &optional FIXEDCASE LITERAL STRING SUBEXP)

Replace text matched by last search with NEWTEXT. Leave point at the end of the replacement text.

If optional second arg FIXEDCASE is non-nil, do not alter the case of the replacement text. Otherwise, maybe capitalize the whole text, or maybe just word initials, based on the replaced text. If the replaced text has only capital letters and has at least one multiletter word, convert NEWTEXT to all caps. Otherwise if all words are capitalized in the replaced text, capitalize each word in NEWTEXT.