They said—

A line with exactly 0 or 1 characters, or a line with a sequence of 1 or 3 or more characters, repeated at least twice

Note—

…or a line with a sequence of 1 or 3 or more characters, repeated at least twice

It should be—

…or a line with a sequence of 2 or more characters, repeated at least twice

The regex in the post will match “abab”. Their original description (line 2 of this comment) will not match “abab”.