How to use a field validation on a text field to insure it is numeric

I need to validate a short text field to insure that both the length is 10 and that no Alpha Characters are enterd.  The reason the field is not defined as a number or integer is due to the fact that the field needs to be able to handle and maitain leading zeros in the string.  I know how to do the Len valdiation, but not sure of how to check for Alpha characters as I do not see any such functions ... any suggestions ?