That's not quite the same, from man apt-cache(8):

search regex... search performs a full text search on all available package lists for the POSIX regex pattern given, see regex(7). It searches the package names and the descriptions for an occurrence of the regular expression and prints out the package name and the short description, including virtual package names.(...) if --names-only is given then the long description is not searched, only the package name is.

Interestingly aptitude search and apt-cache search --names-only doesn't give quite the same results on my box (apart from the fact that aptitude also lists 32bit "i386" packages). But they're almost the same.

As for the names:

    $ aptitude search mbox|grep -v i386 -c
    24

    $ aptitude search mbox|tail -1
    p   yahoo2mbox - Retrieve and store Yahoo! Groups messages