Are there any specific requirements on the number of underbar (underscore) characters used as separators between Object Descriptor (OD) levels? Appendix B.2 isn't explicit about this. The examples seem to imply that two underbars should only be used as a separator before the Simple Data Element level. But other sections of the document use two underbars as separators at other levels. For example, on section 2.4 page 63 we see "270B1__ST01__TransactionSetIdentifierCode"; notice that there are two underbars between the first two levels.
Is it possible to get a formal grammar for Object Descriptors using Augmented Backus–Naur form (ABNF) or something similar? That would ensure we can always parse and validate ODs.
We are writing a companion guide for trading partners and need to make unambiguous references to X12N implementation guide structures.
There are two underbars when there is no Loop ID to be placed between the set ID and the segment ID. Also, when there is no composite placed between the reference designator and the data element name.