A regret letter is a formal document expressing remorse or disappointment over a decision or situation, such as declining a job offer or not participating in an event. It typically begins with a polite greeting, followed by a clear statement of regret regarding the specific matter. The letter should convey appreciation for the opportunity or consideration, acknowledging any positive aspects of the experience. It's essential to keep the tone professional and sincere while providing a brief explanation for the decision, if appropriate. Ending on a positive note can help maintain goodwill and relationships for potential future interactions, reflecting professionalism and respect.