Calculation in one field

i'm new to sql and pl/sql. To practice I was giving an assignment to make a calculator. That part works. But they also want the possibility to type the calculation in the text field and then it needs to work. For example 4+4 (then the = button or enter on your keyboard) or 4+6-3=.

My calculator with buttons works, but not if I type a calculation in the text field. Can anyone help me with this?

This is the code I have in my total:

declare
    l_operator varchar2(1) := :P3_OPERATOR;
    l_value1 number := :P3_VALUE1;
    l_value2 number := :P3_VALUE2;
    l_result number := nvl(:P3_VALUE1,0);
    begin
    case l_operator
    when '+' then
        l_result := l_value1 + l_value2;
    when '-' then
         l_result := l_value1 - l_value2;
    when '*' then
         l_result := l_value1 * l_value2;
    when '/' then
         l_result := l_value1 / l_value2;
    else
        null;
    end case;
    :P3_OPERATOR := null;
    :P3_VALUE2 := null;
    :P3_VALUE1 := l_result;    
    :P3_NUMBERFIELD := l_result;
    end;

with this for als extra for the +, -, * and \ .

   :P12_OPERATOR := '*';
    :P12_NUMBERFIELD := :P12_OPERATOR;

and this is the code for all my number buttons:

 begin
    if :P12_OPERATOR is null then
    :P12_VALUE1 := :P12_VALUE1 || 4;
    :P12_NUMBERFIELD := :P12_VALUE1;
    elsif :P12_OPERATOR is not null then
    :P12_VALUE2 := :P12_VALUE2 || 4;
    :P12_NUMBERFIELD := :P12_VALUE2;
    end if;
    end; 

This is not a typical way to use SQL or PL/SQL (or APEX which it looks like you are also using)!

You could evaluate any expression typed in with code like this:

begin
   execute immediate 'select ' || :P3_NUMBERFIELD || ' from dual' into l_result;
exception
   when others then
      l_result := 'Invalid input';
end;

The exception part is to stop the calculator going wrong if the user types in nonsense like "hello world" instead of an arithmetic expression. The user would need to type in an expression like 4+4 without typing the equals sign, and then press a button to invoke the process to calculate the result.