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BNF

n     A metalanguage is a language used to talk about a language (usually a different one)

n     We can use English as its own metalanguage (e.g. describing English grammar in English)

n     It is essential to distinguish between the metalanguage terms and the object language terms

n     BNF stands for either Backus-Naur Form or Backus Normal Form

n     BNF is a metalanguage used to describe the grammar of a programming language

n     BNF is formal and precise

n     BNF is essential in compiler construction

n     < > indicate a nonterminal that needs to be further expanded, e.g. <variable>

n     Symbols not enclosed in < > are terminals; they represent themselves, e.g. if, while, (

n     The symbol ::= means is defined as

n     The symbol  |  means or; it separates alternatives,

      e.g. <addop> ::= + | -

n     <integer> ::= <digit> | <integer> <digit>
     or
<integer> ::= <digit> | <digit> <integer>

n     "Extended BNF" allows repetition as well as recursion

n     Repetition is often more efficient when using BNF to construct a compiler

n     <digit> ::=
        0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

n     <if statement> ::=
        if ( <condition> ) <statement>
     |  if ( <condition> ) <statement>
        else <statement>

n     <identifier> ::=
        <letter>
     |  <identifier> <letter>
     |  <identifier> <digit>