String_hash.rb

class String

        KEYWORD_REGEX = /(?:|[^%])(%\((\w+)\))/

        def keyreplace! &block
                while match = self.match( KEYWORD_REGEX ) 
                        offsets = match.offset( 1 )
                        replacement = block.call( match[2].to_sym ) || ""
                        self[ offsets[0] ... offsets[1] ] = replacement
                end
                self
        end

        def keyreplace &block
                self.clone.keyreplace! &block
        end

        def & arg
                keyreplace do |k|
                        arg[ k ] or raise ArgumentError.new( "No key in replacement hash: '#{ k.to_s }'" )
                end
        end

end

i = "foo %(name) bar %(fog)"

i.keyreplace { |k| { :name => "hello" }[ k ] }
i & { :name => "world", :fog => "mist" }

"foo %s" % [ "foo" ]