If you will accept evaluating this all at once, here is one way to do it (only works if the rhs is legit MATLAB syntax, which your example isn't):
>> s = 'f(x) = x^2 + 2/(2*x) + 3'
s =
f(x) = x^2 + 2/(2*x) + 3
>> e = find(s=='=',1)
e =
6
>> p = find(s=='(',1)
p =
2
>> f = str2func(['@' s(p:e-1) s(e+1:end)])
f =
@(x)x^2+2/(2*x)+3
>> f(2)
ans =
7.5000
If you really need to evaluate it piecemeal for some reason, that is going to be much more involved because you will need to code up a parser with operator precedences etc.