и так далее... Например, глава 2.4.1. Specifying Program Encoding.Strings are fundamentally different in Ruby 1.8 and Ruby 1.9:
In Ruby 1.8, strings are a sequence of bytes. When strings are used to represent text (instead of binary data), each byte of the string is assumed to represent a single ASCII character. In 1.8, the individual elements of a string are not characters, but numbers—the actual byte value or character encoding.
In Ruby 1.9, on the other hand, strings are true sequences of characters, and those characters need not be confined to the ASCII character set. In 1.9, the individual elements of a string are characters—represented as strings of length 1—rather than integer character codes. Every string has an encoding that specifies the correspondence between the bytes in the string and the characters those bytes represent. Encodings such as the UTF-8 encoding of Unicode characters use variable numbers of bytes for each character, and there is no longer a 1-to-1 (nor even a 2-to-1) correspondence between bytes and characters.
OReilly - The Ruby Programming Language
Видимо плохо ищите.