where < symbol> is a nonterminal , and the expressionAn expression combines numbers, operators, and/or variables. Expressions may be evaluated to values, and may be said to represent those values. The evaluation of an expression is dependent on the definition of the mathematical operators and system of valu consists of sequences of symbols and/or sequences separated by the vertical barVertical bar 'Verticon', or pipe is the name of the ASCII character at position 124 (decimal). The character is depicted as either a solid vertical bar or a vertical bar with a break in the middle (broken bar ). The character is usually depicted as a brok, '|', indicating a choice, the whole being a possible substitutionIn general, substitution is the replacement of one thing with another. There are many specific examples: In organic chemistry, a reaction where one functional group takes the place of another. A major type of substitution is nucleophilic substitution. for the symbol on the left. Symbols that never appear on a left side are terminalThe term Terminal can be used in several way and includes various topics: Usually terminal means forming or pertaining to an end. In Computing, it refers to an electronic or electromechanical hardware device. See Computer terminal. In Telecommunication, ts. Symbols inside brackets [] are optional.
2 Example
As an example, consider this BNF for a USThe United States of America also referred to as the United States U. America č or the States is a federal republic in central North America, stretching from the Atlantic in the east to the Pacific Ocean in the west. It shares land borders with Canada in postal address :
::=
::= | "."
::= [] |
::= []
::= ","
This translates into English as:
- "A postal-address consists of a name-part, followed by a street-address part, followed by a zip-code part. A personal-part consists of either a first name or an initial followed by a dot. A name-part consists of either: a personal-part followed by a last name followed by an optional "jr-part" (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 street address consists of an optional apartment specifier, followed by a street number, followed by a street name. 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."
Note that many things (such as the format of a personal-part, apartment specifier, or ZIP-code) are left unspecified here. If necessary, they may be described using additional BNF rules, or left as abstraction if irrelevant for the purpose at hand.
