Ex post facto law

An "ex post facto" law (Latin for "after the fact" or "from after the action") is a law that changes the legal consequences (or status) of actions that were committed before the law went into effect. In criminal law, an "ex post facto" law may criminalize actions that were legal at the time they were committed, or may make a crime worse by bringing it into a more severe category than it was in when it was committed. 
In criminal law.
It may make the punishment prescribed for a crime, more severe. It may also add new penalties or it may extend a sentence. It may change the rules of evidence in order to make conviction for a crime likelier than it would have been when the deed was committed. , a form of "ex post facto" law, commonly called an "amnesty law", may certain acts. A pardon has a similar effect in a specific case. Other legal changes may lessen or eliminate possible punishments (for example by replacing the death sentence with lifelong imprisonment) ly. Such legal changes are also known by the Latin term in mitius.
US Constitution.
The United States Constitution s "ex post facto" laws. Two clauses in the constitution prohibit "ex post facto". Article 1 Section 9 of the U.S. Constitution states: 'No Bill of Attainder or ex post facto Law shall be passed,'. Section 10 says: 'No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law. . .'
In civil law.
"Ex post facto" laws relate only to criminal laws passed by legislations. It does not apply to civil laws "that affect private rights adversely." In 2003 the US Supreme court noted the difference between civil and penal laws. In the case of Smith v. Doe, the court questioned the constitutionality of the Alaska Sex Offender Registration Act's retroactive requirements. Alaska's Megan's Law was applied to sex offenders before the law went into effect. It required offenders to register with the local police. It also required public notification via the Internet. The court made a legal precedent in deciding the intent of the law was civil and non-punitive. That it was to protect the public saftey by "protecting the public from sex offenders."
Europe.
The European Parliament prohibits "ex post facto" legislation by all member nations. They determined ""Ex post facto" legislation may also violate citizens’ right to effective legal redress and a fair trial..."
In English law, "ex post facto" laws are very rare. One example was the War Crimes Act 1991 by the Parliament of England. It allowed British citizens to be put on trial for war crimes that took place during World War II.