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Douglas Crockford


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2020-01-09

McKeeman Form

    This is an excerpt from Chapter 22 of How JavaScript Works.

McKeeman Form is a notation for expressing grammars. It was proposed
by Bill McKeeman of Dartmouth College. It is a simplified Backus-Naur
Form with significant whitespace and minimal use of metacharacters.

Grammar

We can express the grammar of McKeeman Form in McKeeman Form.

A grammar is a list of one or more rules.

grammar rules

The Unicode code point U+0020 is used as the space. The Unicode code
point U+000A is used as the newline.

space '0020'

newline '000A'

A name is a sequence of letters or _underbar.

name letter letter name

letter 'a' . 'z' 'A' . 'Z' '_'

An indentation is four spaces.

indentation space space space space

Each of the rules is separated by a newline. A rule has a name on one
line, with alternatives indented below it.

rules rule rule newline rules

rule name newline nothing alternatives

If the first line after the name of a rule is "", then the rule may
match nothing.

nothing "" indentation '"' '"' newline

Each alternative is indented on its own line. Each alternative
contains items followed by a newline.

alternatives alternative alternative alternatives

alternative indentation items newline

The items are separated by spaces. An item is a literal or the name
of a rule.

items item item space items

item literal name

literal singleton range exclude '"' characters '"'

Any single Unicode code point except the 32 control codes may be
placed within the single quotes. The hexcode of any Unicode code
point may also be placed within the single quotes. A hexcode can
contain 4, 5, or 6 hexadecimal digits.

singleton ''' codepoint '''

codepoint ' ' . '10FFFF' hexcode

hexcode "10" hex hex hex hex hex hex hex hex hex hex hex hex hex

hex '0' . '9' 'A' . 'F'

A range is specified as a singleton, a .period, and another
singleton. Literal ranges can optionally be followed by -minus sign
and characters to be excluded.

range singleton space '.' space singleton

exclude "" space '-' space singleton exclude space '-' space range
exclude

A character wrapped in "double quote can be any of the Unicode code
points except the 32 control codes and "double quote. The definition
of character shows an example of a codepoint range and exclude.

characters character character characters

character ' ' . '10FFFF' - '"'

JSON

This is the JSON grammar in McKeeman Form.

json element

value object array string number "true" "false" "null"

object '{' ws '}' '{' members '}'

members member member ',' members

member ws string ws ':' element

array '[' ws ']' '[' elements ']'

elements element element ',' elements

element ws value ws

string '"' characters '"'

characters "" character characters

character '0020' . '10FFFF' - '"' - '\' '\' escape

escape '"' '\' '/' 'b' 'f' 'n' 'r' 't' 'u' hex hex hex hex

hex digit 'A' . 'F' 'a' . 'f'

number integer fraction exponent

integer digit onenine digits '-' digit '-' onenine digits

digits digit digit digits

digit '0' onenine

onenine '1' . '9'

fraction "" '.' digits

exponent "" 'E' sign digits 'e' sign digits

sign "" '+' '-'

ws "" '0020' ws '000A' ws '000D' ws '0009' ws