My recommendation would be to use a syntax similar to Excel formula syntax, since a lot of users already know this, and it is a very basic prefix operator notation, which makes it easy to convert this syntax to LISP syntax, for which many parsers (and intepreters) are already available, including examples in the Parse::RecDescent distribution.

Christian Lemburg
Brainbench MVP for Perl
http://www.brainbench.com