Hidden features of Ruby - Stack Overflow

Continuing the "Hidden features of ..." meme, let's share the lesser-known but useful features of Ruby programming language. Try to limit this discussion with core Ruby, without any Ruby on Rails stuff. See also: Hidden features of C#; Hidden features of Java; Hidden features of JavaScript; (Please, just one hidden feature per answer.) Thank you,

http://stackoverflow.com/questions/63998/hidden-features-of-ruby

http://b.hatena.ne.jp/entry/http://stackoverflow.com/questions/63998/hidden-features-of-ruby