- Possessive Quantifiers: http://www.rubular.com/r/4sLa0thZNh
- Named back-references: http://www.rubular.com/r/vmD4oUcZWM
- Callback references (paren group): http://www.rubular.com/r/OC3ONC3TUp
- Callback references (duplicated text): http://www.rubular.com/r/mpI2mrlXe3
http://pragdave.blogs.pragprog.com/pragdave/2008/10/fun-with-ruby-19-regular-expressions.html