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"Bingham, Jay" <Jay.Bingham@hp.com> wrote in message news:<mailman.1034620095.15632.help-gnu-emacs@gnu.org>...

> 
> Here is your function using a string arg-descriptor, that does some very 
> crude numeric validation and inserts a 5 when nothing is entered at the 
> prompt.
> 
> (defun demo (&optional number)
>   "Demo of an     optional argument of the function."
>   (interactive "sEnter a numeric value:")
>   (setq defaultval "5")
>   (if (string= "" number) (setq number defaultval))
>   (if (not (string-match "[:alpha:]" number))
>       (insert (format "%s" number))
>     (message "non-numeric value entered, please re-enter"))
>   )

It works on my GNU Emacs 20.3.1 when modified to:

  (if (string-match "[0-9]+" number)
      (insert (format "%s" number))
    (message "non-numeric value entered, please re-enter"))
  )


But as you said below fails to ask number again when "x" is given
as the value. I am still lookin for a good solution that does validation
and probably displays the default value when it asks the number
first time without ruining single instantiation of the default value.

> If you want a function that prompts for a number until one is entered 
> you will have to use an elisp expression type arg-descriptor and do your 
> own number validation.