s = "hello"
s[0,2] # "he"
s[-1,1] # "o": returns a string, not the character code ?o
s[0,0] # "": a zero-length substring is always empty
s[0,10] # "hello": returns all the characters that are available
s[s.length,1] # "": there is an empty string immediately beyond the end
s[s.length+1,1] # nil: it is an error to read past that
s[0,-1] # nil: negative lengths don't make any sense