I'm making a gui calculator, how to convert string to operators?

Another method is to use a dictionary storing functions:

--Translate string into mathematical expression
local prefixesFunctions = {
    ["/"] = function (number1,number2) 
        return number1/number2 
    end;
	["-"] = function (number1,number2) 
        return number1 - number2 
    end;
	["*"] = function (number1,number2) 
        return number1*number2 
    end;
	["+"] = function (number1,number2) 
        return number1+number2 
    end;
}

From this other post, with a similar gui calculator project: