It is not about the code line by line, but the functionality that OP created for their employer. And yes it is not clear-cut in the sense that in Oracle vs. Google it was AFAIK decided that the idea of the toString Method does not fall under copyright. However, a software that fills a specific need for a company and is then re-implemented/released by an employee? You can bet your ass you are in for at least a lengthy battle in court.