[[NSCharacterSet lowercaseLetterCharacterSet] characterIsMember:[password characterAtIndex:i]]//当前字符是小写

[[NSCharacterSet uppercaseLetterCharacterSet] characterIsMember:[password characterAtIndex:i]]//当前字符是大写

[[NSCharacterSet characterSetWithCharactersInString:@"0123456789"] characterIsMember:[password characterAtIndex:i]]//当前字符是数字

[[NSCharacterSet characterSetWithCharactersInString:@"`~!?@#$€£¥§%^&*()_+-={}[]:\";.,<>'•\\|/"] characterIsMember:[password characterAtIndex:i]]//当前字符是特殊符号