Short answers, pulled from the story.
Prince Regent George IV took the title of Prince Regent in 1811. This occurred because his father, King George III, had descended into madness. The period defined fashion, architecture, and social conduct for nearly a decade.
Queen Catherine de Medici was the Italian noblewoman who became Queen of France and ruled as a regent during the reigns of her sons Francis II, Charles IX, and Henry III. She orchestrated the St. Bartholomew's Day Massacre of 1572 to eliminate Protestant leaders and secure Catholic dominance. Her regency was a calculated campaign of manipulation that made her the most influential woman in Europe.
The Regency Period in Brazil began in 1831 when a five-year-old boy named Pedro II was crowned Emperor. A provisional triumvirate of regents took the oath of office to govern on his behalf until he came of age. This period was marked by political instability and regional revolts.
The Roman Catholic primate, the Archbishop of Gniezno, held the office of interrex in the Polish-Lithuanian Commonwealth. This formalized role existed from 1569 to 1795 during the long interregnum that followed the death of a monarch. The interrex held the power of the state until a new monarch could be elected.
The two Captains Regent in San Marino are elected for a six-month term as joint heads of state. This system has been in place since the 13th century and allows the republic to maintain its independence for over 700 years. The dual regency is a unique feature where power is shared between two individuals.
Prem Tinsulanonda became the oldest regent of any nation in 2016 at the age of 96. He served as the regent for Rama X of Thailand until the new king could be crowned. This formalized role allowed the state to maintain stability during the transition period.