Replace the prefix placeholder
        :class:`~aeneas.globalconstants.PPV_OS_TASK_PREFIX`
        with ``custom_id`` and return the resulting string.

        :rtype: string

def _replace_placeholder(self, string, custom_id):
        """
        Replace the prefix placeholder
        :class:`~aeneas.globalconstants.PPV_OS_TASK_PREFIX`
        with ``custom_id`` and return the resulting string.

        :rtype: string
        """
        if string is None:
            return None
        self.log([u"Replacing '%s' with '%s' in '%s'", gc.PPV_OS_TASK_PREFIX, custom_id, string])
        return string.replace(gc.PPV_OS_TASK_PREFIX, custom_id)