Editing Backus–Naur form (section)

==Variants==

=== EBNF ===
{{Main|Extended Backus–Naur form}}
There are many variants and extensions of BNF, generally either for the sake of simplicity and succinctness, or to adapt it to a specific application. One common feature of many variants is the use of [[regular expression]] repetition operators such as <code>*</code> and <code>+</code>. The [[extended Backus–Naur form]] (EBNF) is a common one.

Another common extension is the use of square brackets around optional items. Although not present in the original ALGOL 60 report (instead introduced a few years later in [[International Business Machines|IBM]]'s [[PL/I]] definition), the notation is now universally recognised.

=== ABNF ===
{{Main|ABNF}}
[[Augmented Backus–Naur form]] (ABNF) and Routing Backus–Naur form (RBNF)<ref>[http://tools.ietf.org/html/rfc5511 RBNF].</ref> are extensions commonly used to describe [[Internet Engineering Task Force]] (IETF) [[protocol (computing)|protocol]]s.

[[Parsing expression grammar]]s build on the BNF and [[regular expression]] notations to form an alternative class of [[formal grammar]], which is essentially [[analytic grammar|analytic]] rather than [[generative grammar|generative]] in character.

=== Others ===
Many BNF specifications found online today are intended to be human-readable and are non-formal.  These often include many of the following syntax rules and extensions:
* Optional items enclosed in square brackets: <code>[&lt;item-x&gt;]</code>.
* Items existing 0 or more times are enclosed in curly brackets or suffixed with an asterisk (<code>*</code>) such as <code>&lt;word&gt; ::= &lt;letter&gt; {&lt;letter&gt;}</code> or <code>&lt;word&gt; ::= &lt;letter&gt; &lt;letter&gt;*</code> respectively.
* Items existing 1 or more times are suffixed with an addition (plus) symbol, <code>+</code>, such as <code>&lt;word&gt; ::= &lt;letter&gt;+</code>.
* Terminals may appear in bold rather than italics, and non-terminals in plain text rather than angle brackets.
* Where items are grouped, they are enclosed in simple parentheses.