With a block given, calls the block with each +String+ value
returned by successive calls to String#succ;
the first value is +self+, the next is <tt>self.succ</tt>, and so on;
the sequence terminates when value +other_string+ is reached;
returns +self+:

  a = []
  'a'.upto('f') {|c| a.push(c) }
  a # => ["a", "b", "c", "d", "e", "f"]

  a = []
  'Ж'.upto('П') {|c| a.push(c) }
  a # => ["Ж", "З", "И", "Й", "К", "Л", "М", "Н", "О", "П"]

  a = []
  'よ'.upto('ろ') {|c| a.push(c) }
  a # => ["よ", "ら", "り", "る", "れ", "ろ"]

  a = []
  'a8'.upto('b6') {|c| a.push(c) }
  a # => ["a8", "a9", "b0", "b1", "b2", "b3", "b4", "b5", "b6"]

If argument +exclusive+ is given as a truthy object, the last value is omitted:

  a = []
  'a'.upto('f', true) {|c| a.push(c) }
  a # => ["a", "b", "c", "d", "e"]

If +other_string+ would not be reached, does not call the block:

  '25'.upto('5') {|s| fail s }
  'aa'.upto('a') {|s| fail s }

With no block given, returns a new Enumerator:

  'a8'.upto('b6') # => #<Enumerator: "a8":upto("b6")>

Related: see {Iterating}[rdoc-ref:String@Iterating].