My copy of Black's (6th Ed.) has no such entry, but it does have an entry for Exemplum.
"In the civil law, copy; a written authorized copy. This word is also used in the modern sense of example - ad exemplum constituti singulares non trahi, exceptional things must not be taken for examples."
Regillium and Regulium are not given. Did you misread? Should it be Regulum ("rule")?