Emacs: Universal argument (C-u) in a function

I would like to use C-u in a function (for instance, regexp) where calling it with C-u has a different effect. How can I do this in Emacs? The documentation doesn't show how to do this with Emacs Lisp.

(defun test ()
  (interactive)
  (align-regexp)) ; I would like to add the C-u prefix to this.