(Latin: rules of law)
In general, rules of legal interpretation.
In a more specific sense, regulae juris are certain legal axioms found in the Decretals of Gregory IX and the Decretals of Boniface VIII.
There are 99 in all, referred to as R.J.
Some instances will make the matter clear.
"Once bad, always presumed to be bad"; this applies only "in the same crime, e.g., if a man has been proven a perjurer once, he is excluded from giving testimony again, but he would not be considered a homicide from the fact that he was a perjurer.
"There is no injury done to one who knowingly and freely consents"; this suggests that no one is injured if he knowingly and willingly gives up a right which he is allowed to surrender, but does not apply in case he is morally incapable of giving it up.
New Catholic Dictionary