Sentence:  Judge Short had gone to town, and Farrar was off for a three days' cruise up the lake. I was bitterly <WSD>regretting</WSD> I had not gone with him when the distant notes of a coach horn reached my ear, and I descried a four-in-hand winding its way up the inn road from the direction of Mohair. 
Meaning of the word marked with <WSD> tags:  To feel sorry about (a thing that has or has not happened), afterthink: to wish that a thing had not happened, that something else had happened instead.