The presumption of regularity is a legal principle regarding the actions taken by a government. What the principle holds is that any action that the government takes is presumed to be legal. If a person believes that some action of the government or of a government official is illegal, that person must show that the presumption of legality is incorrect. That is, the person must show that the government has acted illegally.
The presumption of regularity holds that any action taken by the government is legal until evidence is provided that shows that it is not legal.