1 U.S.C. § 109 — Repeal of statutes as affecting existing liabilities

This section makes sure that repealing or letting a law expire doesn't wipe out old penalties.

Repealed statutes. When Congress repeals a statute, that repeal doesn't release or extinguish any penalty, forfeiture, or liability that someone already incurred under that statute — unless the new, repealing Act expressly says it does. If the repealing Act doesn't say that, the old, repealed statute is still treated as being in force for one purpose: letting the government properly pursue a lawsuit or prosecution to enforce that penalty, forfeiture, or liability.

Expired temporary statutes. The same rule applies when a temporary statute simply expires on its own, rather than being repealed. The expiration doesn't release or extinguish a penalty, forfeiture, or liability that already arose under it, unless the temporary statute itself expressly says otherwise. Absent that express statement, the expired statute is still treated as remaining in force for the purpose of sustaining a proper action or prosecution to enforce that penalty, forfeiture, or liability.