在UITextField中过滤字符

我想要过滤掉指定的字符，但是没实现，不知道哪错了？？请大家帮忙，谢谢

代码：

   - (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string{
if ([textField isEqual: txtCountry]) {

NSCharacterSet *cs = [[NSCharacterSet characterSetWithCharactersInString:LEAGELNUM] invertedSet];
NSString *filtered = [[string componentsSeparatedByCharactersInSet:cs]     componentsJoinedByString:@""];
BOOL basicTest = [string isEqualToString:filtered];
return basicTest;
}
}