Practice - Chapter 7. Ex 25:

If L is any language. Let s(L) be set of all strings made by taking strings in L whose length is a multiple of 3 and shrinking it by removing its middle third, that is s(L) = {ab | for some c, |a| = |c| = |b| and abc L}. Show that if L is a regular language then s(L) is not necessarily regular.