validate username characters and length

lib/twhois.rb

@@ -12,12 +12,20 @@ module Twhois
   LOOKUP_HOST = "api.twitter.com"
   LOOKUP_PATH = "/1/users/show.json?screen_name="
   
+  class InvalidUsername < StandardError; end
+  
   # Lookup a Twitter user by their username.
   def self.lookup(username)
+    raise InvalidUsername, "Username is invalid" unless valid_username?(username)
     res = Net::HTTP.start(LOOKUP_HOST) { |http| http.get(LOOKUP_PATH + username) }
     if res.code == '200'
       User.new(JSON.parse(res.body))
     end
   end
   
+  def self.valid_username?(username)
+    return false if username.match(/^[a-zA-Z0-9_]{1,15}$/).nil?
+    return true
+  end
+  
 end