A cowardly Lower Manhattan subway slasher was sentenced to 21 years prison today for a random and brutal box-cutter assault on a young woman’s face.

“My life has been changed forever,” Yael Leopold said an impassioned victim impact statement, describing the emotional aftermath of being attacked by homeless, drug-addled Edwin Santiago as she waited on a J-train platform in the Chambers Street platform in January, 2003.

“My whole life was turned upside down,” said Leopold, who suffered four deep slashes and nearly lost her thumb fending off the attack.

Santiago, 40, had first been convicted in 2004, but that conviction was overturned by appellate judges who ruled that defense lawyers had been improperly barred from calling an expert witness to testify about the fallibility of eyewitness testimony.

Today, Leopold’s face barely shows a scar. A University of Massachusetts student at the time of the attack, she has healed emotionally as well — returning to the city to work as an elementary teacher in the city school system.

“He showed no remorse,” assistant district attorney Matthew Bogdanos told Manhattan Supreme Court Justice Marcy Kahn, who called it a “horrendous and horrible crime.”

“It’s a crime without reason, a crime without pity, a crime without motive,” the prosecutor said. “It is the pure picture of violence.”

Santiago, meanwhile, continues to insist that he’s the wrong guy. “I have nothing but sympathy for the victim, but I was not there,” he told the judge.

“I’ve never seen this lady before in my life.”

After the sentence, Manhattan DA Cyrus Vance issued a statement praising the victim for returning to New York to teach its children, thereby “not letting this terrifying episode define her experience in New York.”