cell format: only numbers (no date) with any amount of decimals

Hi,
I need a cell to restrict the input:
-Only numbers are allowed.
-No date posible.
-Any amount of decimals (they must all be shown in the cell).

I tried using the data validation and using the IsNumber() to restrict any non numeral input. The problem with this approach is that if the user enters a date; it apprears as a date format (eg: "5.May"). I'm using an european excel, where the decimal separator is a comma instead of a point; so if a user accidentaly types "5.5" instead of "5,5"; the cell will show "5.May".

I also tried the cell format/number/number format. The problem in here is that I dont know how many decimal positions will the input number have; and I need them all to be shown.

Is there any way to accomplish this?

P.S. Unfortunatelly I am not allowd to use macros to solve this issue.

Thanks a million.