Попробуйте это
NSRange start = [string rangeOfString:@"http://www.youtube.com/"];
NSRange end = [string rangeOfString:@"\" "];
int rangeLength = (int)(end.location - start.location);
NSString *hrefString = [[NSString alloc] initWithString:[string substringWithRange:NSMakeRange(start.location, rangeLength)]];
NSLog(@"%@",hrefString);