replace-regexp-in-string bug?

[Lennart Borgman]

I want to create a regexp out of an unknown string with spaces. I am not 
sure about the number of spaces in the actual string to match so I want 
to do a replacement like this:

   (replace-regexp-in-string "\s-+" "\s-+" "some space" nil t)

However this returns "some space" instead of "some\s-+space" which I 
expected. Is this a bug or am I misunderstanding the arguments in some way?