The first suggestion would be find out what expressions you want to support. Is it just the four basic arithmetic ops? What representations do you need of numbers (eg., base ten? Scientific notation like 8E6? Rationals? Arbitrary precision reals? Complex numbers?). Do you need precedence to work implicitly (eg., is "32 + 9/5 * x" "32 + ((9/5) * x)"? Or "((32 + 9)/5) * x"? Something else)? Do you need to allow arbitrary functions that you had defined earlier?

Depending on how simple your needs, you may be able to use a simple regular expression to do the parsing for you, or you may need something more involved.

Comment onRe: Inputting Math Functions