Sorry if I was mistaken, it seemed to me you were being a smart aleck.
MixedString = MixedString.replace(/\D/g,'');
You'd call that function just like a VBScript function. So, in addition to the way Oracleguy showed above, you could also see if something extracted from a database was only numeric like:
If myString <> ExtractNumbers(myString) Then
'It's not numeric!