Keep it simple.

#
# Pad to 8 characters
#

for I in 0 9 99 999 9999 99999 999999 9999999 99999999 ; do
    PAD="00000000$I"
    echo "${PAD##${PAD%%????????}}"
done

Output:

00000000
00000009
00000099
00000999
00009999
00099999
00999999
09999999
99999999