HOWTO use a Factory

Hi all,

i have some questions/problems about "Factories".

My origin is a constructor which needs a string in special format.

An example:

class Example_t {

    public:
                Example_t(std::string &aString);
    ...
}

My first idea was to check "aString" IN the constructor with
"boost::regex" AND to throw an exception if "aString" was bad.

Now I know it's bad to throw an exception in a constructor.

But what else?

How can I AVOID elegant a construction of an instance of "Example_t"
if "aString" is in bad format?

I've read a lot at the net but I'm still unhappy. At some other forum
I got the information to use something like that:

class Validator
{
  public:
  virtual bool validate(std::string) = 0;
};

class A
{
  private:
  A();
};

class Factory
{
  public:
  static A createA(std::string, *Validator);
  static B createB(std::string, *Validator);
};

But I don't understand how to AVOID a construction with classes above
if the string is in bad format?

Thanks

Goran