Q: I need code to parse and evaluate expressions.
A: Two available packages are ``defunc,'' posted to comp.sources.misc in December, 1993 (V41 i32,33), to alt.sources in January, 1994, and available from sunsite.unc.edu in pub/packages/development/libraries/defunc-1.3.tar.Z, and ``parse,'' at lamont.ldgo.columbia.edu. Other options include the S-Lang interpreter, available via anonymous ftp from amy.tch.harvard.edu in pub/slang, and the shareware Cmm (``C-minus-minus'' or ``C minus the hard stuff''). See also questions 18.16 and 20.6.
There is also some parsing/evaluation code in Software Solutions in C (chapter 12, pp. 235-55).