When responding to an invitation expressing regrets, it is best to match the tone of the invitation. That is, an engraved invitation requires a formal response while invitations received by email, phone or in a quick note can be responded to informally.
One of the biggest social faux pas is failure to R.S.V.P. to an invitation. Whether the function is to be attended or not, letting the host or hostess know as soon as possible is proper etiquette. According to the writings of etiquette expert Emily Post, anyone who receives an invitation stating "R.S.V.P." is obligated to reply. A formal response expressing regrets may read:
"Mr. and Mrs. John Smith regret they will not be able to attend the wedding of Susan Wright and Max Marley."