literial to expression

i want to convert a literial string to an expression then evaluate it. for example;

"a+b+c/d*e(f-g)"

wants to be

output = a+b+c/d*e(f-g)

where the vars a, b, c etc have been defined.

so the ideal would be:

output = magicfunction("a+b+c/d*e(f-g)");

thanks for anyhelp