Army:
* Colonel: This is the most common title for the officer in charge of a regiment.
* Lieutenant Colonel: In some cases, a Lieutenant Colonel might be in charge of a smaller regiment or battalion.
Other Branches:
* Commanding Officer (CO): This is a general term used across branches for the officer in charge.
* Regimental Commander: This title is sometimes used in the US Marines and Navy for the officer in charge of a regiment.
Historically:
* Regimental Sergeant Major (RSM): This was traditionally the senior enlisted soldier in a regiment and responsible for discipline and administration.
Important Note: The specific title and responsibilities of the officer in charge of a regiment will vary based on the specific organization and its mission.