If you are using Ubuntu 13.10, then you can install ack-grep 2.04.2 using the Ubuntu Software Center. The ack-grep package is from the universe repository.
To give an example of what ack-grep can do, here is a command to display just five lines from somewhere in the middle of the bash manual, which is larger than 300Kb, and therefore it is inconvenient to find a referenced line number in the bash manual by scrolling down in the terminal, but it's easy to do with ack-grep:
man bash | ack-grep . --lines=4581-4585
...gives the following snippet from the bash manual as output:
Print the absolute pathname of the current working directory.
The pathname printed contains no symbolic links if the -P option
is supplied or the -o physical option to the set builtin command
is enabled. If the -L option is used, the pathname printed may