Textbox that accepts all the characters but returns only numbers-Collection of common programming errors

I need the code to filter the data entered in a textbox. Although it accepts all the characters during runtime, the code should remove all the strings and alpha numeric characters except the numbers (which would be my output). I tried the following code but guess it won’t do:

a = Textbox1.text
Dim value As Decimal = CDec(Regex.Replace(a, "[\D]", ""))