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

I arrived here looking for a way to detect if my function had been called with C-u. This is how you do it:

(defun my-function ()
 (interactive)
  (if (equal current-prefix-arg nil) ; no C-u
   ;; then
    (message "my-function was called normally")
   ;; else
    (message "my-function was called with C-u")))

What the original poster was asking is how to call another function with C-u, from inside his/her function. I post this as a clarification to @codyChan's comment above, in the hope that it might help others.

(defun my/test ()
  (interactive)
  (let ((current-prefix-arg 4)) ;; emulate C-u
    (call-interactively 'align-regexp) ;; invoke align-regexp interactively
    )
  )

Hope that helps.