Quiz question on string matching

[Dirk Laurie <dirk.laurie@...>]

  Find a pattern with no captures such that putting it in parentheses
  may change what it matches.

More precisely:

   Find strings `str` and `pat` such that `testanswer(str,pat)` prints
   'true', where

testanswer = function(str,pat)
if pat:match"[()]" then return end
print(str:match(pat) ~= str:match('(' .. pat .. ')'))
end