Validate a URL string in my iphone app

Question

Hi,

I am getting really frustrated and I cant seem to check if the user has entered a valid url or not. This is what I have tried:

NSString *str = [NSString stringWithFormat:@"%@", [myurl.text stringByAddinPercentEscapesUsingEncoding:NSUTF8StringEncoding]]; 

NSURL *aurl = [NSURL URLWithString:urlStr]; 
if(aurl == nil){
//Invalid url
}