Finding out whether a string is numeric or not

Here’s one way that doesn’t rely on the limited precision of attempting to parse the string as a number:

NSCharacterSet* notDigits = [[NSCharacterSet decimalDigitCharacterSet] invertedSet];
if ([newString rangeOfCharacterFromSet:notDigits].location == NSNotFound)
{
    // newString consists only of the digits 0 through 9
}

See +[NSCharacterSet decimalDigitCharacterSet] and -[NSString rangeOfCharacterFromSet:].