...Source: old_str, Start: MIDDLE, Stop: str_len(old_str));

There are usually help files...

...Stop = str_len(old_str));

or

 s = substring(old_str, MIDDLE, str_len(old_str...

...be far superior.

First/Last - intuitive
Start/Len - intuitive
First/Last+1 - might be easy...

...sizeof:

int[] array = {3,4,5};
int len = array.length; /* returns 3 */
int size = array...

...extend it
int len = myArray.length;
(myArray = new int[len*2])[0..len-1] = myArray...