Who wrote the following: "For of all sad words of tongue and pen, the saddest are these: It might have been!"?

"Maud Muller" is a poem from 1856 written by John Greenleaf Whittier (1807–1892). It is about a beautiful maid named Maud Muller. One day, while harvesting hay, she meets a judge from the local town. Each is smitten with the other. The judge thinks that he would like to be a local farmer married to Maud, while she thinks that she would like to be the wife of the wealthy judge. Neither voices these thoughts, however, and both the judge and the maiden move on. The judge marries a woman of wealth whose love for him is based on his riches. Maud Muller marries a young uneducated farmer. Throughout the rest of their lives, each remembers the day of their meeting and remorsefully reflects on what might have been.

This poem contains the well-known quotation: "For of all sad words of tongue or pen, the saddest are these: It might have been!".