SYNTAX = . PRODUCTION = IDENTIFIER "=" EXPRESSION ".". EXPRESSION = TERM . TERM = FACTOR . FACTOR = IDENTIFIER | LITERAL | "" | "" | "". IDENTIFIER = letter . LITERAL = """" character """".
The equals sign indicates a production. The element on the left is defined to be the combination of elements on the right. A production is terminated by a full stop.
Repetition is denoted by curly brackets, e.g., stands for ε | a | aa | aaa |....
Optionality is expressed by square brackets, e.g.,b stands for ab | b.
Parentheses serve for groupings, e.g.,c stands for ac | bc.
We take these concepts for granted today, but they were novel and even controversial in 1977. Wirth later incorporated some of the concepts into extended Backus–Naur form. Notice that letter and character are left undefined. This is because numeric characters may be included in both definitions or excluded from one, depending on the language being defined, e.g.: digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9". upper-case = "A" | "B" | … | "Y" | "Z". lower-case = "a" | "b" | … | "y" | "z". letter = upper-case | lower-case.
If charactergoes on to include digit and other printable ASCII characters, then it diverges even more from letter, which one can assume does not include the digit characters or any of the special characters.
Another example
The syntax of BNF can be represented with WSN as follows, based on translating the BNF example of itself: syntax = rule . rule = opt-whitespace "<" rule-name ">" opt-whitespace "::=" opt-whitespace expression line-end. opt-whitespace = . expression = list . line-end = opt-whitespace EOL | line-end line-end. list = term . term = literal | "<" rule-name ">". literal = """" text """" | "'" text "'".
This definition appears overcomplicated because the concept of "optional whitespace" must be explicitly defined in BNF, but it is implicit in WSN. Even in this example, text is left undefined, but it is assumed to mean "ASCII-character ". Notice how the kludge"<" rule-name ">" has been used twice because text was not explicitly defined. One of the problems with BNF which this example illustrates is that by allowing both single-quote and double-quote characters to be used for a literal, there is an added potential for human error in attempting to create a machine-readable syntax. One of the concepts migrated to later metasyntaxes was the idea that giving the user multiple choices made it harder to write parsers for grammars defined by the syntax, so computer languages in general have become more restrictive in how a quoted-literal is defined.
