Here are some examples:
* The regnant queen held a grand ball in her honor.
* The regnant emperor was known for his wisdom and justice.
You might also see "regnant" used to describe something that is dominant or prevalent, such as a particular style or idea:
* The regnant fashion trend was for bold colors and patterns.
"Regnant" is an adjective and often used in formal writing, especially when discussing royalty or historical figures.