:
s/[1-9]/&_/g
y/123456789/012345678/
s/_0/7___/
t
:2
s/0_\{16\}/_0/
t2
:3
s/._/&0123456789ABCDEF /
s/\(.\)_.*\1\(.\).* /\2/
t3
s/^0//