isearch-lax question error?

[Ergus]

Hi:

I am trying to create a lax isearch interactive that use ".*?" but
independent from the default one very useful as well.

I already have isearch-lax-whitespace and search-whitespace-regexp to
the defaults.

And in my function I only want to change search-spaces-regexp in order
to make the command temporarily more flexible.

so far I have something simple like this:

(defun my/isearch-forward ()
  (interactive)
  (let ((search-spaces-regexp ".*?"))        ;; I alternated this with 
search-whitespace-regexp
    (call-interactively 'isearch-forward)))

However this does not behave as expected; it changes nothing compared to
the default behavior.

But If I set (defcustom isearch-lax-whitespace ".*?") then I get the
right behavior in the normal isearch-forward.

What I am missing here?

Thanks in advance,
Ergus