Is it true an ex post facto law is a law that makes an act illegal after the act has been committed?

Yes, an ex post facto law cannot impose punishment on a person who committed an act before it was illegal. This includes increasing the severity of the punishment from what it was when the crime was committed. Ex post facto applies to criminal law and not civil law as interpreted by the U.S. Supreme Court. Also, some laws, for example the sex offenders registry, are considered a regulatory device for public safety and not a punitive action.

no ex-post facto law after the fact. no ex-post facto law after the fact.