For a single character, I like to provide an integer value of the number of bytes (or characters, depending) offset from the beginning of the string. For a sub

For a single character, I like to provide an integer value of the number of bytes (or characters, depending) offset from the beginning of the string. For a substring, I provide the same starting offset plus another integer for the length (again, in characters or bytes depending on context). I think you'll find these are very popular approaches.