E.g., input your function as a MATLAB syntactically correct string, then use the str2func function to form a function handle. E.g.,
>> s = input(' Function of x = ','s')
Function of x = 2*x+1
s =
2*x+1
>> fun = str2func(['@(x)' s])
fun =
@(x)2*x+1
>> fun(1)
ans =
3
>> fun(2)
ans =
5
>> fun(3)
ans =
7
