$ sudo apt-get install apt-file
$ sudo apt-file update
$ apt-file search autoexpect
Apt-file comes in handy many times. One thing to note is that you don't need to run ‘apt-file update' very often (once every release upgrade) since the file names don't change that much, just the file contents.
For the record, if you want to figure out which installed package a file belongs to, use ‘dpkg -S'. This doesn't always work for config files, and files in /var, since those are generated after the package is extracted.