ios 字符串中部分字符串加下划线

UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(10, 100, 300, 100)];
label.numberOfLines = 3;
NSMutableAttributedString *str = [[NSMutableAttributedString alloc]initWithString:[NSString stringWithFormat:@"剩余 200 天"]];
NSRange contentRange = {3,[str length] - 5};
[str addAttribute:NSUnderlineStyleAttributeName value:[NSNumber numberWithInteger:NSUnderlineStyleSingle] range:contentRange];
label.attributedText = str;
[self.view addSubview:label];