InfoPath Dev

I have textbox which should take only numerics. First i did this thing by validation i.e User gives input, if it has any other character than numeric it gives error. But my client wants that textbox should limit the user. User should not be able to enter any other character other than numerics. (just like we can limit textbox control to specific number of characters) PS: I want to work this textbox just like we can use MaskedTextBox in C#

The only way to do this is to add a separate field for each character and make each field a dropdown with the 0-9 numbers in it. Best of luck!