I'm sure it's the same as URL itself, here is from wiki:
List of allowed URL characters Unreserved (May be encoded but it is not necessary): A B C D E F G H I J K L M N O P Q R S T U V W X Y Z a b c d e f g h i j k l m n o p q r s t u v w x y z 0 1 2 3 4 5 6 7 8 9 - _ . ~
Reserved (Have to be encoded sometimes) ! * ' ( ) ; : @ & = + $ , / ? % # [ ]