Right, and about a quarter contain the same digit at least twice in a row, which may be the "double digit" the grandparent referred to. $ pins() { seq -f '

Right, and about a quarter contain the same digit at least twice in a row, which may be the "double digit" the grandparent referred to. $ pins() { seq -f '%04.0f' 0 9999; } $ pins | egrep -c '(.)\1' 2710 $ pins | egrep -c '(.).*\1' 4960 $