How to set NSString’s background cornerRadius on iOS7

You can do this by using an UITextView with a subclass of NSLayoutManager, that override -fillBackgroundRectArray:count:forCharacterRange:color:.
Just a little sample how to this :

@implementation ViewController
- (void)viewDidLoad
{
    [super viewDidLoad];

    // setup text handling
    NSTextStorage *textStorage = [[NSTextStorage alloc] initWithString:@"Lorem ipsum dolor sit er elit lamet, consectetaur cillium adipisicing pecu, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum. Nam liber te conscient to factor tum poen legum odioque civiuda."];

    // use our subclass of NSLayoutManager
    MyLayoutManager *textLayout = [[MyLayoutManager alloc] init];

    [textStorage addLayoutManager:textLayout];

    NSTextContainer *textContainer = [[NSTextContainer alloc] initWithSize:self.view.bounds.size];

    [textLayout addTextContainer:textContainer];
    UITextView *textView = [[UITextView alloc] initWithFrame:CGRectMake(0,20,self.view.bounds.size.width,self.view.bounds.size.height-20)
                                               textContainer:textContainer];
    [self.view addSubview:textView];

    // set some background color to our text
    [textView.textStorage setAttributes:[NSDictionary dictionaryWithObject:[UIColor blueColor] forKey:NSBackgroundColorAttributeName] range:NSMakeRange(22, textView.text.length - 61)];
}
@end

@interface MyLayoutManager : NSLayoutManager
@end

- (void)fillBackgroundRectArray:(const CGRect *)rectArray count:(NSUInteger)rectCount forCharacterRange:(NSRange)charRange color:(UIColor *)color
{
     CGFloat halfLineWidth = 4.; // change this to change corners radius

     CGMutablePathRef path = CGPathCreateMutable();

     if (rectCount == 1
         || (rectCount == 2 && (CGRectGetMaxX(rectArray[1]) < CGRectGetMinX(rectArray[0])))
        )
    {
        // 1 rect or 2 rects without edges in contact

         CGPathAddRect(path, NULL, CGRectInset(rectArray[0], halfLineWidth, halfLineWidth));
         if (rectCount == 2)
            CGPathAddRect(path, NULL, CGRectInset(rectArray[1], halfLineWidth, halfLineWidth));
    }
     else
    {
        // 2 or 3 rects
         NSUInteger lastRect = rectCount - 1;

         CGPathMoveToPoint(path, NULL, CGRectGetMinX(rectArray[0]) + halfLineWidth, CGRectGetMaxY(rectArray[0]) + halfLineWidth);

         CGPathAddLineToPoint(path, NULL, CGRectGetMinX(rectArray[0]) + halfLineWidth, CGRectGetMinY(rectArray[0]) + halfLineWidth);
         CGPathAddLineToPoint(path, NULL, CGRectGetMaxX(rectArray[0]) - halfLineWidth, CGRectGetMinY(rectArray[0]) + halfLineWidth);

         CGPathAddLineToPoint(path, NULL, CGRectGetMaxX(rectArray[0]) - halfLineWidth, CGRectGetMinY(rectArray[lastRect]) - halfLineWidth);
         CGPathAddLineToPoint(path, NULL, CGRectGetMaxX(rectArray[lastRect]) - halfLineWidth, CGRectGetMinY(rectArray[lastRect]) - halfLineWidth);

         CGPathAddLineToPoint(path, NULL, CGRectGetMaxX(rectArray[lastRect]) - halfLineWidth, CGRectGetMaxY(rectArray[lastRect]) - halfLineWidth);
         CGPathAddLineToPoint(path, NULL, CGRectGetMinX(rectArray[lastRect]) + halfLineWidth, CGRectGetMaxY(rectArray[lastRect]) - halfLineWidth);

         CGPathAddLineToPoint(path, NULL, CGRectGetMinX(rectArray[lastRect]) + halfLineWidth, CGRectGetMaxY(rectArray[0]) + halfLineWidth);

         CGPathCloseSubpath(path);
    }

    [color set]; // set fill and stroke color

     CGContextRef ctx = UIGraphicsGetCurrentContext();
     CGContextSetLineWidth(ctx, halfLineWidth * 2.);
     CGContextSetLineJoin(ctx, kCGLineJoinRound);

     CGContextAddPath(ctx, path);
     CGPathRelease(path);

     CGContextDrawPath(ctx, kCGPathFillStroke);
}
@end