Editing Extended Backus–Naur form (section)

==Extensibility==
According to the ISO 14977 standard EBNF is meant to be extensible, and two facilities are mentioned. The first is part of EBNF grammar, the special sequence, which is arbitrary text enclosed with question marks.  The interpretation of the text inside a special sequence is beyond the scope of the EBNF standard.  For example, the space character could be defined by the following rule:

<syntaxhighlight lang="ebnf">
 space = ? ASCII character 32 ?;
</syntaxhighlight>

The second facility for extension is using the fact that parentheses in EBNF cannot be placed next to identifiers (they must be concatenated with them). The following is valid EBNF:

<syntaxhighlight lang="ebnf">
 something = foo, ( bar );
</syntaxhighlight>

The following is ''not'' valid EBNF:

<syntaxhighlight lang="ebnf">
 something = foo ( bar );
</syntaxhighlight>

Therefore, an extension of EBNF could use that notation. For example, in a [[Lisp (programming language)|Lisp]] grammar, function application could be defined by the following rule:

<syntaxhighlight lang="ebnf">
 function application = list( symbol, { expression } );
</syntaxhighlight>