The plus sign matches the existence of the preceding character or character class one time or more, so it must exist at least once:
$ echo "tt" | awk '/to+t/{print $0}'$ echo "tot" | awk '/to+t/{print $0}'$ echo "toot" | awk '/to+t/{print $0}'$ echo "tt" | sed -r -n '/to+t/p'$ echo "tot" | sed -r -n '/to+t/p'$ echo "toot" | sed -r -n '/to+t/p'
The first example doesn't have an o character, and that's why it's the only example that has no match.
Also, we can use the plus sign with the character class:
$ echo "tt" | awk '/t[oa]+t/{print $0}'$ echo "tot" | awk '/t[oa]+t/{print $0}'$ echo "toot" | awk '/t[oa]+t/{print $0}$ echo "tt" ...
