iOS中限制文本框输入内容

这段代码是我工作需要只能够限制文本框只能够输入中文英文，不能够输入数字和字符的代码

#define NMUBERS @"0123456789./*-+~!@#$%^&()_+-=,./;'[]{}:<>?`"

-(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string{

    

    //不能多于6个字符

    if (self.text.text.length >=20) {

        return NO;

    }

    

    //只能输入英文或中文

    NSCharacterSet * charact;

    charact = [[NSCharacterSet characterSetWithCharactersInString:NMUBERS]invertedSet];

    

    NSString * filtered = [[string componentsSeparatedByCharactersInSet:charact]componentsJoinedByString:@""];

    

    BOOL canChange = [string isEqualToString:filtered];

    if(canChange) {

        UIAlertView * alert = [[UIAlertView alloc] initWithTitle:@"提示" message:@"请输入英文或中文"

                                                        delegate:nil

                                               cancelButtonTitle:@"确定"

                                               otherButtonTitles:nil];

        [alert show];

        return NO;

    }

    

    return YES;

}