NSCharacterSet在苹果官方文档中的description:
An NSCharacterSet object represents a set of Unicode-compliant characters. NSString and NSScanner objects use NSCharacterSetobjects to group characters together for searching operations, so that they can find any of a particular set of characters during a search. The cluster’s two public classes, NSCharacterSet and NSMutableCharacterSet, declare the programmatic interface for static and dynamic character sets, respectively.

NSCharacterSet在苹果官方文档中的description:
An NSCharacterSet object represents a set of Unicode-compliant characters. NSString and NSScanner objects use NSCharacterSetobjects to group characters together for searching operations, so that they can find any of a particular set of characters during a search. The cluster’s two public classes, NSCharacterSet and NSMutableCharacterSet, declare the programmatic interface for static and dynamic character sets, respectively.