There is no exception for negative numbers, because negative numbers are not exceptional when parsing for numbers.
Don't try to handle with with an exception, or in the catch block. Try to parse the string to get a number. If it fails with a parse exception, tell the user. If it doesn't fail, then after parsing, check to see if the number is negative. If it's negative, then tell the user. You'd do this in the try block not the catch block.