vsep

VSEP is able to parse expressions like:

(foo + bar + quz)
(foo + bar(1, 2 (foo + bar)))
foo('some_string',bar, 1.001e1)

It can then "solve" these expression, getting any variables or function return values from various callbacks supplied to it. For the & and | operators (logical and and or, respectively) it supports lazy evaluation. VSEP can also be passed a callback returning the "difficulty" of getting a certain variable or function, which it can use to simplify the expression as lazily as possible.

VSEP is written in C; it strives for efficiency, readable and thoroughly documented code, a high degree of configurabilty and lack of global variables.