Grammar Syntax

Grammar Syntax

The reference page. If you'd rather learn by example, start with Your First Grammar.

Tokens and rules

Kind	How to spot it	What it's for
Token	Name is entirely UPPERCASE (NUMBER, LOGICAL_OP)	Lexer-level pieces: identifiers, numbers, operators
Rule	Lowercase name (expr)	Parser-level structure built from tokens and other rules
Nested rule	Written as #name: inside another rule	A rule scoped to its parent

Every definition ends with ;.

🚧 Not yet supported: using a rule inside a token. This is planned, see Roadmap and Status.

Capturing: @

@ marks the part of a production you want to capture. Anything you don't mark isn't captured, which keeps your AST small and your lexer fast.

NUMBER:
  @ [0-9]+ ('.' @ [0-9]+)?
  @{decimal, floating}
;

The data block: {@}

{@} assigns one single AST node to the production. The value can be either:

• @, meaning the first (and only) thing you captured, or
• a Python-style expression.

ID:
  @ ([a-zA-Z_][a-zA-Z0-9_]*)
  {@}
;

Naming captures: @{...}

When a production captures several things, @{a, b, c} names them. Put @ before each element you want, then list the names in the block. The names become the fields of the generated AST node.

#compare:
    @ arithmetic (@ COMPARE_OP @ arithmetic)*
    @{first, op, sequence}
;

Note: @name will produce an ast node name with no control directed to @{}. @ name will direct. Single space matters here.

Nested rules: #name

#name: defines a rule that lives inside another rule. A nested rule can be regular or advanced.

A regular (non-advanced) rule can, in turn, contain more rules inside it. These go right after the rule's content, followed by its data block or its fail block.

Nesting is handy for things like precedence levels, where every level is a small rule that belongs to one parent. See the full expr example in Your First Grammar.

Advanced rules use braces and executable clauses. For example, COMMENT { match '#' [^\n\0]*; ctx.skip() } recognizes a comment and tells the lexer to discard it. See examples/yaml/grammar.isc for indentation handling with ctx.emit, shared state, and ctx.error. The embedded action language is a supported Python subset, not arbitrary Python.

Operators you've already seen

Syntax	Meaning
'text'	A literal string
[0-9], [^"]	Character classes (and negated classes)
( ... )	Grouping
*, +, ?	Zero or more, one or more, optional
|	Alternation

Semantic actions

Grammars can include Python-style pseudocode for semantic actions. This is what makes context-sensitive lexing possible.

🚧 LR/LALR accept advanced rules containing ordinary match clauses, but reject embedded Python actions and conditional matches. LR(*) currently rejects builds. See Algorithms and Limitations.

For grammar modules and external parser imports, see Modules and External Parsers. See Counted Repetition, Modifiers, Templates And Inheritance, and Lexer Semantic Builder for their current syntax and behavior.

Fail blocks

Fail blocks define custom error messages and recovery. They're still limited, so they have their own page: Error Recovery.