Check out the sed and awk tutorials here: http://www.grymoire.com/Unix/Sed.html

sed in particular is really amazing in what it can do, very simply. (Note, depending on your system, you might want to get GNU sed, for "extended" regex support.)

Those pages do a great job breaking down the basic behavior and explaining pitfalls. They're skim-able, and pretty easily searchable after than, so you can come back and remind yourself of the thing you learned last week.