Friday, December 11, 2015

BNF Notation For Syntax

BNF or Backus Normal Form is a computer science notation technique that's useful in describing the syntax of a language. This Wikipedia article is an easy example that shows how it works:

<postal-address> ::= <name-part> <street-address> <zip-part>

      <name-part> ::= <personal-part> <last-name> <opt-suffix-part> <EOL> 
                    | <personal-part> <name-part>

  <personal-part> ::= <initial> "." | <first-name>

 <street-address> ::= <house-num> <street-name> <opt-apt-num> <EOL>

       <zip-part> ::= <town-name> "," <state-code> <ZIP-code> <EOL>

<opt-suffix-part> ::= "Sr." | "Jr." | <roman-numeral> | ""
    <opt-apt-num> ::= <apt-num> | ""
  
BNF excerpt from Wikipedia

  • A postal address consists of a name-part, followed by a street-address part, followed by a zip-code part.
  • A name-part consists of either: a personal-part followed by a last name followed by an optional suffix (Jr., Sr., or dynastic number) and end-of-line, or a personal part followed by a name part (this rule illustrates the use of recursion in BNFs, covering the case of people who use multiple first and middle names and/or initials).
  • A personal-part consists of either a first name or an initial followed by a dot.
  • A street address consists of a house number, followed by a street name, followed by an optional apartment specifier, followed by an end-of-line.
  • A zip-part consists of a town-name, followed by a comma, followed by a state code, followed by a ZIP-code followed by an end-of-line.
  • A opt-suffix-part consists of a suffix, such as "Sr.", "Jr." or a roman-numeral, or an empty string (i.e. nothing).
  • A opt-apt-num consists of an apartment number or an empty string (i.e. nothing).
  • The '::=' means that the symbol on the left must be replaced with the expression on the right.
Bullet list explanation from Wikipedia

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