# irb >> h = {:banana => 100, :tomato => 120, :apple => 200} => {:apple=>200, :banana=>100, :tomato=>120} >> h.invert => {100=>:banana, 200=>:apple, 120=>:tomato}

a = %w(banana orange apple) #=> ["banana", "orange", "apple"] r = Regexp.union(a.map{|e| /\A#{Regexp.escape(e)}\z/}) #=> /(?-mix:\Abanana\z)|(?-mix:\Aorange\z)|(?-mix:\Aapple\z)/ "apple" =~ r #=> 0 "bananana" =~ r #=> nil