mjhjmtu

Pressing Ctrl+A, H, lets me log output of the screen command. However, the output file is too large.
Is it possible to apply a grep-like filter so that I can control what gets logged?

For example, I might wish to log only lines which contain the phrase foo bar.

Interesting question. I just skimmed through man screen so I am not absolutely sure whether it is possible to achieve without using external tools. However, one can use a named pipe and a combination of tail and grep to do this:

$ mkfifo /tmp/fifo/fifo
$ tail -f /tmp/fifo/fifo | grep --line-buffered bar >> /tmp/DONE

Inside screen do:

logfile /tmp/fifo/fifo

and start logging. After finishing logging only lines that contained bar will be saved in /tmp/DONE. As it states in man fifo:

A FIFO special file (a named pipe) is similar to a pipe, except that
it is accessed as part of the file system. It can be opened by
multiple processes for reading or writ- ing. When processes are
exchanging data via the FIFO, the kernel passes all data internally
without writing it to the file system. Thus, the FIFO special file has
no con- tents on the file system; the file system entry merely serves
as a reference point so that processes can access the pipe using a
name in the file system.

That means that /tmp/fifo/fifo takes no space on disk. This solution worked for me but I don't know what negative side-effects or shortcomings it may have.

EDIT:

I just noticed that they recommend using fifo in man script:

-f, --flush Flush output after each write. This is nice for telecooperation: one person does 'mkfifo foo; script -f foo', and
another can supervise real-time what is being done using 'cat foo'.

In case you didn't know, script is an utility that also records terminal session.