The Extended Backus-Naur form (EBNF) is any variation on the basic Backus-Naur form (BNF) metasyntax notation with (some of) the following additional constructs:
- square brackets "[..]" surrounding optional items,
- suffix "*" for Kleene closure (a sequence of zero or more of an item), suffix "+" for one or more of an item, curly brackets enclosing a list of alternatives,
- super/subscripts indicating between n and m occurrences.
All these constructs can be expressed in plain BNF using extra productions and have been added for readability and succinctness.
There is an International standard (ISO 14977) that defines an EBNF. A draft is freely available from http://www.cl.cam.ac.uk/~mgk25/iso-ebnf.html. This draft document has some mistakes in it. Throughout the text, as well as in the comments of the examples, `meta-identifiers' are `written as one or more words joined together by hyphens' (as stated in section 4). On the other hand, all examples (except the comments) use spaces to separate several words.
This article (or an earlier version of it) contains material from FOLDOC, used with permission.