Meta-grammar for context-free grammars
Backus–Naur Form's syntax can be represented with a BNF itself. Here is an example taken from this Wikipedia article:

&lt;syntax&gt;         ::= &lt;rule&gt; | &lt;rule&gt; &lt;syntax&gt;
&lt;rule&gt;           ::= &lt;opt-whitespace&gt; "&lt;" &lt;rule-name&gt; "&gt;"
                     &lt;opt-whitespace&gt; "::=" &lt;opt-whitespace&gt; 
                     &lt;expression&gt; &lt;line-end&gt;
&lt;opt-whitespace&gt; ::= " " &lt;opt-whitespace&gt; | ""
&lt;expression&gt;     ::= &lt;list&gt; |
                     &lt;list&gt; &lt;opt-whitespace&gt; "|" &lt;opt-whitespace&gt; &lt;expression&gt;
&lt;line-end&gt;       ::= &lt;opt-whitespace&gt; &lt;EOL&gt; | &lt;line-end&gt; &lt;line-end&gt;
&lt;list&gt;           ::= &lt;term&gt; | &lt;term&gt; &lt;opt-whitespace&gt; &lt;list&gt;
&lt;term&gt;           ::= &lt;literal&gt; | "&lt;" &lt;rule-name&gt; "&gt;"
&lt;literal&gt;        ::= '"' &lt;text&gt; '"' | "'" &lt;text&gt; "'"


"" denotes the empty string. The article provides some more details on undefined here rules for &lt;rule-name&gt;, etc.