So... are these files keyed? Are they sorted? I presume that they're text files with variable length records... what are the line endings?
What counts as a match? Should it be case sensitive? Does it have to be the entire line, or is one of the fields acting as a primary key? If so, are there any escaping rules we should know about before trying to split the fields, e.g. don't split on | if it's preceeded by a \.