What is BNF?
Backus-Naur Form (BNF) is a popular notation for describing the syntax
of context-free grammars. Although there are many variants and extensions
of BNF, it essentially consists of the following notation:
| ::= | the production symbol for declaring production rules |
| | | the disjunction symbol for separating alternate production rules |
| " | double-quotation marks for delimiting literal strings |
| [ ] | square-brackets for delimiting optional terms |
| ( ) | parentheses for grouping terms |
For example, the following production rule is from the JAVA programming language:
if_statement ::=
"if" "(" expression ")" statement
[ "else" statement ] ;
Because of its power and simplicity, Backus-Naur Form is often used to describe the syntax of programming languages, database query languages, and scripting languages. BNF has high familiarity in compiler development communities. There are many variations and extensions of BNF in use today, including Extended Backus-Naur Form (EBNF) and Augmented Backus-Naur Form (ABNF). Although these extensions can themselves be expressed in BNF, they provide additional notation for expressing constructs such as repeating and optional terms.
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