There is a written-down specification, but not a formal grammar (I assume you implied the latter): https://orgmode.org/worg/org-syntax.html

The standalone project, org-parser is pretty good! https://github.com/200ok-ch/org-parser (usable from Javascript, Java).

See also: "Formal Specification and Programmatic Parser for Org-mode" https://www.reddit.com/r/emacs/comments/s0zvlh/formal_specif...