def coerce(cls, string):
        """Create a :class:`Requirement` object from a given package spec."""
        match = re.match(cls.spec_regex, string)
        if not match:
            raise InvalidRequirementSpecError("could not parse requirement")
        package = match.group(1)
        if all(match.group(2, 3)):
            version = "".join(match.group(2, 3))
        else:
            version = None
        return cls(package, version)
def get_date(self, p_tag):
        """ Given a date tag, return a date object. """
        string = self.tag_value(p_tag)
        result = None

        try:
            result = date_string_to_date(string) if string else None
        except ValueError:
            pass

        return result