If you're going to do it in the Crystal Report Formula Workshop, you'd probably be best off using the RIGHT function. The parms are the string and the length of characters you want to take. There is also a LEFT and a MID function that can be used for substringing. Here are some links that might help you.
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If you're going to do it in the code before you pass data to the report, you'll have to provide more information such as what language you programming in.