How to determine if there is a number in a string?

Hello,

I am building a report that needs to be filtered based on a range of aircraft serial number.  The serial number is a text field, because many serial numbers are a combo of numbers and letters.  I am running into problems casting these numbers from a CHARACTER to an INTEGER type, because on occasion serial numbers are entered outside of the proper format by mistake.  These few entries are causing my sub-string functions to not properly isolate just the number portion of the serial number.  I am almost positive this is why I am getting a run time error. Is