Я бы использовал метод NSString rangeOfCharacterFromSet:
для обработки вашей проверки:
NSString *str = myTextField.text;
BOOL isValid = ([str length] > 6 &&
[str rangeOfCharacterFromSet:[NSCharacterSet letterCharacterSet]].location != NSNotFound &&
[str rangeOfCharacterFromSet:[NSCharacterSet decimalDigitCharacterSet]].location != NSNotFound &&
[str rangeOfCharacterFromSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]].location == NSNotFound);