The Ruby %r{ } expression

In a model there is a field validates :image_file_name, :format => { :with => %r{\.(gif|jpg|jpeg|png)$}i It looks pretty odd for me. I am aware that this is a regular expression. But I would like: to know what exactly it means. Is %r{value} equal to /value/ ? be able to replace it with normal Ruby regex operator /some regex/ or =~. Is this possible?