5.1.5 Grammar Notation

 

Terminal symbols of the lexical and string grammars, and some of the terminal symbols of the syntactic grammar, are shown in fixed width font, both in the productions of the grammars and throughout this specification whenever the text directly refers to such a terminal symbol. These are to appear in a program exactly as written. All nonterminal characters specified in this way are to be understood as the appropriate Unicode character from the ASCII range, as opposed to any similar-looking characters from other Unicode ranges.

Nonterminal symbols are shown in italic type. The definition of a nonterminal is introduced by the name of the nonterminal being defined followed by one or more colons. (The number of colons indicates to which grammar the production belongs.) One or more alternative right-hand sides for the nonterminal then follow on succeeding lines. For example, the syntactic definition:

WithStatement :

with ( Expression ) Statement

states that the nonterminal WithStatement represents the token with, followed by a left parenthesis token, followed by an Expression, followed by a right parenthesis token, followed by a Statement. The occurrences of Expression and Statement are themselves nonterminals. As another example, the syntactic definition:

ArgumentList :

AssignmentExpression

ArgumentList , AssignmentExpression

states that an ArgumentList may represent either a single AssignmentExpression or an ArgumentList, followed by a comma, followed by an AssignmentExpression. This definition of ArgumentList is recursive, that is, it is defined in terms of itself. The result is that an ArgumentList may contain any positive number of arguments, separated by commas, where each argument expression is an AssignmentExpression. Such recursive definitions of nonterminals are common.

The subscripted suffix "opt", which may appear after a terminal or nonterminal, indicates an optional symbol. The alternative containing the optional symbol actually specifies two right-hand sides, one that omits the optional element and one that includes it. This means that:

VariableDeclaration :

Identifier Initialiser_opt

is a convenient abbreviation for:

VariableDeclaration :

Identifier
Identifier Initialiser

and that:

IterationStatement :

for ( ExpressionNoIn_opt ; Expression_opt ; Expression_opt ) Statement

is a convenient abbreviation for:

IterationStatement :

for (; Expression_opt ; Expression_opt ) Statement

for ( ExpressionNoIn ; Expression_opt ; Expression_opt ) Statement

which in turn is an abbreviation for:

IterationStatement :

for (;; Expression_opt ) Statement

for (; Expression ; Expression_opt ) Statement

for ( ExpressionNoIn ;; Expression_opt ) Statement

for ( ExpressionNoIn ; Expression ; Expression_opt ) Statement

which in turn is an abbreviation for:

IterationStatement :

for (;;) Statement

for (;; Expression ) Statement

for (; Expression ;) Statement

for (; Expression ; Expression ) Statement

for ( ExpressionNoIn ;;) Statement

for ( ExpressionNoIn ;; Expression ) Statement

for ( ExpressionNoIn ; Expression ;) Statement

for ( ExpressionNoIn ; Expression ; Expression ) Statement

so the nonterminal IterationStatement actually has eight alternative right-hand sides.

If the phrase "[empty]" appears as the right-hand side of a production, it indicates that the production's right-hand side contains no terminals or nonterminals.

If the phrase "[lookahead ∉ set]" appears in the right-hand side 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. The set can be written as a list of terminals enclosed in curly braces. For convenience, the set can also be written as a nonterminal, in which case it represents the set of all terminals to which that nonterminal could expand. For example, given the definitions

DecimalDigit :: one of

0 1 2 3 4 5 6 7 8 9

DecimalDigits ::

DecimalDigit
DecimalDigits DecimalDigit

the definition

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.

If the phrase "[no LineTerminator here]" appears in the right-hand side of a production of the syntactic grammar, it indicates that the production is a restricted production: it may not be used if a LineTerminator occurs in the input stream at the indicated position. For example, the production:

ReturnStatement :

return [no LineTerminator here] Expression_opt ;

indicates that the production may not be used if a LineTerminator occurs in the program between the return token and the Expression.

Unless the presence of a LineTerminator is forbidden by a restricted production, any number of occurrences of LineTerminator may appear between any two consecutive tokens in the stream of input elements without affecting the syntactic acceptability of the program.

When the words "one of" follow the colon(s) in a grammar definition, they signify that each of the terminal symbols on the following line or lines is an alternative definition. For example, the lexical grammar for ECMAScript contains the production:

NonZeroDigit :: one of

1 2 3 4 5 6 7 8 9

which is merely a convenient abbreviation for:

NonZeroDigit

1
2
3
4
5
6
7
8
9

When an alternative in a production of the lexical grammar or the numeric string grammar appears to be a multi-character token, it represents the sequence of characters that would make up such a token.

The right-hand side of a production may specify that certain expansions are not permitted by using the phrase "but not" and then indicating the expansions to be excluded. For example, the production:

Identifier ::

IdentifierName but not ReservedWord

means that the nonterminal Identifier may be replaced by any sequence of characters that could replace IdentifierName provided that the same sequence of characters could not replace ReservedWord.

Finally, a few nonterminal symbols are described by a descriptive phrase in roman type in cases where it would be impractical to list all the alternatives:

SourceCharacter ::

any Unicode character