File.open(filename).grep(/string/)
This loads the whole file into memory (slurps the file). You should avoid file slurping when dealing with large files. That means loading one line at a time, instead of the whole file.
File.foreach(filename).grep(/string/)
It’s good practice to clean up after yourself rather than letting the garbage collector handle it at some point. This is more important if your program is long-lived and not just some quick script. Using a code block ensures that the File object is closed when the block terminates.
File.foreach(filename) do |file|
file.grep(/string/)
end