Can't be sure, but it sounds like what you are looking at are files using \n as the segment terminator and '*' as the element separator. With \n as the segment terminator, it's natural to just read the segments as separate lines.

These are good choices for the terminator, even recommended by the standards, IIRC. You're in luck in that your partners haven't used non-printable characters so this data is easy to read by just eyeball inspection. Oh, if you're trying to parse out the elements (the data between the '*'s), remember to quote the '*' properly in the split command. That's one pitfall of using '*', but an easy one to overcome.

The 'BEG', 'N1', 'N3', 'PO1' are called Segment Identifiers. Each field in those segments is defined in terms of allowable values (alpha, numeric, special tables of values, etc.), but there is a lot of latitude on how to use these standards. This latitude is typically nailed down in ICs (Implementation Conventions). In this discussion I have some links to some sample ICs used by the Government and others. These Sample ICs are actually based on the ASC X12 documents.

Ask around and see if your trading partners have written up ICs. It may well be the case that they haven't, in which case you have to reverse engineer them from their use. Getting this right can be complicated.