maybe you can describe a little more what you are trying to achieve? Your "function" has no input, so you could basically evaluate the string:
str = '((5/5)+5)+(5*((5*5)-5))';
value = eval(str);
If you indeed have a function, e.g.
then Per's suggestion of using the "@" and str2func works fine:
fcn = str2func(['@(x) ' str]);