dpkg-awk - 1.2+nmu2 main

This script can parse the dpkg database files. It can do regular
expressions on the fields, and only get what you want. It can also be
made to output certain fields. As an added bonus, there is an option
to sort the output.
.
'dpkg-awk "Status: .* installed$" -- Package Version Status' will
output all installed packages, with only the listed fields.
'dpkg-awk -f=/var/lib/dpkg/available "Package:^[aA].*" -- Package Version'
will output all available packages that start with the letter 'A.'