Extended Backus-Naur form

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The Extended Backus-Naur form (EBNF) is an extension of the basic Backus-Naur form (BNF) metasyntax notation as defined by ISO-14977.

1. The following conventions are used:

Each meta-identifier of Extended BNF is written as one or more words joined together by hyphens;
A meta-identifier ending with “-symbol” is the name of a terminal symbol of Extended BNF.

2. The normal character representing each operator of Extended BNF and its implied precedence is (highest precedence at the top):

* repetition-symbol
- except-symbol
, concatenate-symbol
| definition-separator-symbol
= defining-symbol
; terminator-symbol

3. The normal precedence is over-ridden by the following bracket pairs:

´  first-quote-symbol            first-quote-symbol  ´
"  second-quote-symbol          second-quote-symbol  "
(* start-comment-symbol          end-comment-symbol *)
(  start-group-symbol              end-group-symbol  )
[  start-option-symbol            end-option-symbol  ]
{  start-repeat-symbol            end-repeat-symbol  }
?  special-sequence-symbol   special-sequence-symbol ?

As examples, the following syntax-rules illustrate the facilities for expressing repetition:

aa = "A";
bb = 3 * aa, "B";
cc = 3 * [aa], "C";
dd = {aa}, "D";
ee = aa, {aa}, "E";
ff = 3 * aa, 3 * [aa], "F";
gg = 3 * {aa}, "D";

Terminal-strings defined by these rules are as follows:

aa: A
bb: AAAB
cc: C AC AAC AAAC
dd: D AD AAD AAAD AAAAD etc.
ee: AE AAE AAAE AAAAE AAAAAE etc.
ff: AAAF AAAAF AAAAAF AAAAAAF
gg: D AAAD AAAAAAD etc.

When Niklaus Wirth was developing Pascal, he simplified Backus-Naur Form to create EBNF.

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. This is now freely available electronically as a zipped pdf file from ISO.

The W3C used a different EBNF to specify the XML syntax.

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References

Niklaus Wirth: What can we do about the unnecessary diversity of notation for syntactic definitions? CACM, Vol. 20, Issue 11, November 1977, pp. 822-823.
Roger S. Scowen: Extended BNF — A generic base standard. Software Engineering Standards Symposium 1993.

Publicly available (zipped) document: link ISO IEC 14977 E

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