A Backus–Naur Form (BNF) annotated version of the PowerShell grammar can be found in the first edition of Bruce Payette's book Windows PowerShell in Action. Appendix C containing the grammar is available in PDF form on the publisher's site. https://www.manning.com/books/windows-powershell-in-action
The second edition of Windows PowerShell in Action does not contain a BNF.
Is there a current PowerShell BNF diagram available?