JavaScript 2.0 Grammar Notation

This page explains the formal notation used in the parser grammar and the lexer grammar and their associated semantics. Please refer to the notation page for a description of the informal notation used on the other pages.

Basic Rule Syntax

Each LR(1) parser grammar and lexer grammar rule consists of a nonterminal, a , and one or more expansions of the nonterminal separated by vertical bars (|). The expansions are usually listed on separate lines but may be listed on the same line if they are short. An empty expansion is denoted as «empty».

SampleList

«empty»

| ... Identifier

| SampleListPrefix

| SampleListPrefix , ... Identifier

This rule states that the nonterminal SampleList can represent one of four kinds of sequences of input tokens:

Input tokens are characters (and the special End placeholder) in the lexer grammar and lexer tokens in the parser grammar. Spaces separate input tokens and nonterminals from each other. An input token that consists of a space character is denoted as «SP». Other non-ASCII or non-printable characters are denoted by also using « and », as described on the main notation page.

Lookahead Constraints

If the phrase "[lookahead set]" appears in the expansion of a production, it indicates that the production may not be used if the immediately following input terminal is a member of the given set. That set can be written as a list of terminals enclosed in curly braces. For convenience, set can also be written as a nonterminal, in which case it represents the set of all terminals to which that nonterminal could expand.

DecimalDigit 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

DecimalDigits

DecimalDigit

| DecimalDigits DecimalDigit

LookaheadExample

n [lookahead {1, 3, 5, 7, 9}] DecimalDigits

| DecimalDigit [lookahead {DecimalDigit}]

matches either the letter n followed by one or more decimal digits the first of which is even, or a decimal digit not followed by another decimal digit.

These lookahead constraints do not make the grammars more theoretically powerful than LR(1), but they do allow these grammars to be written more simply. The semantic engine compiles grammars with lookahead constraints into parse tables that have the same format as those produced from ordinary LR(1) or LALR(1) grammars.

Parametrized Rules

Many rules in the grammars occur in groups of analogous rules. Rather than list them individually, these groups have been summarized using the shorthand illustrated by the example below:

introduce grammar arguments a and b. If these arguments later parametrize the nonterminal on the left side of a rule, that rule is implicitly replicated into a set of rules in each of which a grammar argument is consistently substituted by one of its variants. For example, the sample rule

AssignmentExpression^a,b

ConditionalExpression^a,b

| LeftSideExpression^a = AssignmentExpression^normal,b

| LeftSideExpression^a CompoundAssignment AssignmentExpression^normal,b

AssignmentExpression^{normal,allowIn}

ConditionalExpression^{normal,allowIn}

| LeftSideExpression^normal = AssignmentExpression^{normal,allowIn}

| LeftSideExpression^normal CompoundAssignment AssignmentExpression^{normal,allowIn}

AssignmentExpression^normal,noIn

ConditionalExpression^normal,noIn

| LeftSideExpression^normal = AssignmentExpression^normal,noIn

| LeftSideExpression^normal CompoundAssignment AssignmentExpression^normal,noIn

AssignmentExpression^{initial,allowIn}

ConditionalExpression^{initial,allowIn}

| LeftSideExpression^initial = AssignmentExpression^{normal,allowIn}

| LeftSideExpression^initial CompoundAssignment AssignmentExpression^{normal,allowIn}

AssignmentExpression^initial,noIn

ConditionalExpression^initial,noIn

| LeftSideExpression^initial = AssignmentExpression^normal,noIn

| LeftSideExpression^initial CompoundAssignment AssignmentExpression^normal,noIn

AssignmentExpression^{normal,allowIn} is now an unparametrized nonterminal and processed normally by the grammar.

Some of the expanded rules (such as the fourth one in the example above) may be unreachable from the grammar's starting nonterminal; these are ignored.

Special Lexer Rules

A few lexer rules have too many expansions to be practically listed. These are specified by descriptive text instead of a list of expansions after the .

Some lexer rules contain the metaword except. These rules match any expansion that is listed before the except but that does not match any expansion after the except. All of these rules ultimately expand into single characters. For exaple, the rule below matches any single UnicodeCharacter except the * and / characters:

NonAsteriskOrSlash UnicodeCharacter except * | /