I encourage you to re-read the article: you check that the number does not match the regex:
> Is 7 prime?
>
> To know this, the function first generates “1111111” (from “1” \* 7) and tries to
> see if that string does not match /^1?$|^(11+?)\1+$/. If there is no match, then
> the number is prime.
>
> Notice that the regular expression has two parts (separated with the vertical bar |).
>
> The first part is /^1?$/ is trivial and matches with beginning of line (^), an
> optional 1 (1?) and end of line ($) which implies that it matches either the
> empty string or “1”. This simply indicates that calling that function with n==0
> or n==1 will correctly return false (as the “1” \* n will match with the first
> part of the regular expression)