# CJam, PCRE, length 9, [cracked](http://codegolf.stackexchange.com/a/112749/8478)

 ^[a-z~]*$

Example output for 4:

 [[1 2 3 4] [8 7 6 5] [9 10 11 12] [16 15 14 13]]

Now `{|}` are banned, too.