Extract part of a string with s.substr(start, length). First parameter is starting index, second is how many characters. s.substr(0, 3) returns first three characters. Original unchanged.
Omit length and substr(start) returns everything from start to end. Useful for suffixes or string tails without counting remaining characters. Out-of-range parameters behave sensibly.
If length exceeds remaining, you get everything available. If start equals length, you get empty. Starting beyond length throws an exception.