def doi(self) -> str:
        """
        Generate a valid Digital Object Identifier (DOI).
        Format: 10.{4-9 digits}/{alphanumeric string}
        Eg: 10.1000/xyz123

        :sample:
        """
        prefix = "10"
        registrant = str(self.generator.random.randint(1000, 99999999))
        suffix = self.generator.bothify("?#?#?##").lower()

        return f"{prefix}.{registrant}/{suffix}"

        Generate a valid Digital Object Identifier (DOI).
        Format: 10.{4-9 digits}/{alphanumeric string}
        Eg: 10.1000/xyz123

        :sample:
        
faker/providers/doi/__init__.py