Which lines from Robert Frost’s "The Road Not Taken” most convey a tone of regret?

And looked down one as far as I could
To where it bent in the undergrowth;
Though as for that the passing there
Had worn them really about the same,
Yet knowing how way leads on to way,
I doubted if I should ever come back.
I took the one less traveled by,
And that has made all the difference.