For a full overview of XLParser, read the original paper: A Grammar for Spreadsheet Formulas Evaluated on Two Large Datasets.
You can generate API documentation using Doxygen and the Doxyfile in api/Doxyfile
Read this paper (v1.1.2) or this thesis (v1.2.1) for full details. The grammar can also be interfered from the source code. Also available are the EBNF grammar (PDF, PNG, updated v1.2.1) and token list (PDF, PNG, updated v1.2.1).