I think this might be the easiest and safest way to do what you want in Ruby 1.9.x (sprintf doesn’t support reference by name in 1.8.x): use Kernel.sprintf feature of “reference by name”. Example:
>> mystring = "There are %{thing1}s and %{thing2}s here."
=> "There are %{thing1}s and %{thing2}s here."
>> vars = {:thing1 => "trees", :thing2 => "houses"}
=> {:thing1=>"trees", :thing2=>"houses"}
>> mystring % vars
=> "There are trees and houses here."