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)

I agree that the article formatting is a bit misleading.