Considering the syntactic specification of simple arithmetic expressions, its context-free grammar in BNF could be
G = ( Σ, N, P, S )
= ( {+, -, *, (, ), number}, {<exp>, <op>}, P, <exp> )
where P consists of
<exp> ::= <exp> <op> <exp> | ( <exp> ) | number
<op> ::= + | – | *
The grammar is context-free because only single nonterminals occur on the left sides of the rules.
The hyperlink gives the BNF of Pascal.
Extended BNF: EBNF
EBNF, such as the EBNF of Pascal, includes the following two constructs:
- Repetition using { } such as
<stmt-sequence> ::= <stmt> ; <stmt-sequence> | <stmt>
<stmt> ::= s
⇓
<stmt-sequence> ::= { <stmt> ; } <stmt>
<stmt> ::= s
- Option using [ ] such as
<statement> ::= <if-stmt> | other
<if-stmt> ::= if ( <exp> ) <statement>
| if ( <exp> ) <statement> else <statement>
<exp> ::= 0 | 1
⇓
<statement> ::= <if-stmt> | other
<if-stmt> ::= if ( <exp> ) <statement>
[ else <statement> ]
<exp> ::= 0 | 1