Try the lettersPattern() function:
pat = lettersPattern creates a pattern that matches text composed of one or more of letters. lettersPattern is Unicode tolerant.
pat = digitsPattern creates a pattern that matches text composed of one or more digit characters. digitsPattern is Unicode tolerant.
Using those (there are other pattern functions by the way), you can determine whether the user entered letters, numerical digits, or a combination of them into the edit text box.