BNF is Backus Normal Form (or Backus–Naur Form) and is a standard way to define a grammar for a syntax. It makes it easy for parsers to digest data. I was thinking of writing some erlang code for genealogy and wanted to parse my gedcom for the data. Easiest way is using BNF and I assume all the existing .ged parsers have buried in them somewhere the BNF for GED. Is it available open-source anywhere?
While there may be an official standard GED grammar, there are many nonstandard tags that are now widely used. Therefore, using BNF may or may not be the "easiest way".
Nevertheless, a quick google brings up this Gedcom 5.5.1 grammar file.