M-x replace-regexp uppercases words in replacement text based on the first letter present in the line

Question

So, I have the following buffer:

1234
Red
blue
green
yellow

# colors

orange

Now, if I run M-x replace-regexp I get odd results. If I have the search string: ^\(.+\)$ and the replacement string: example string:\1 I get the following output:

example string:1234
Example String:Red  
example string:blue
example string:green
example string:yellow

example string:# colors

example string:orange

As can be seen, I get Example String:Red instead of the expected example string:Red. What's the deal with this behavior?

Answer 1

C-h f replace-regexp says:

Preserve case in each match if case-replace and case-fold-search are non-nil and REGEXP has no uppercase letters.

Either set replace-case to nil before you run the replacement command or write a new command that binds case-replace to nil and then calls replace-regexp. Something like this:

(defun my-replace ()
 "..."
 (interactive)
 (let ((case-replace  nil))
   (replace-regexp "^\\(.+\\)$" "example string:\\1")))

See node Regexp Replace of the Emacs manual.

Answer 2

This is caused by the case-replace option. If you disable that option, the replacement case will be as you entered it for all instances.

The purpose of the option is more to maintain the original case for replacements like red -> green in which case:

red
Red
RED

would become

green
Green
GREEN

If you disable that option, the replacements would be:

green
green
green