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What is Grammar-Kit?

Grammar-Kit is an IntelliJ IDEA plugin by JetBrains for language plugin developers. It transforms BNF grammar definitions into working parsers, PSI classes, and lexer definitions, replacing hand-written parsing code with generated, readable output. The IntelliJ Platform SDK documentation explicitly recommends Grammar-Kit for parser and PSI generation.

Grammar-Kit takes a .bnf grammar file as input and generates a PsiBuilder-based recursive descent parser, element type constants, and PSI (Program Structure Interface) classes. It also generates JFlex lexer definitions (.flex files) from the same grammar. You write the grammar, and Grammar-Kit produces the code that turns source text into a structured syntax tree your IDE plugin can work with.

Use cases

Grammar-Kit handles a range of parsing tasks for IntelliJ-based IDEs:

  • Programming languages: production plugins for Rust, Dart, Erlang, Clojure, Elm, Elixir, Perl, and Haxe all use Grammar-Kit. JetBrains' own Dart plugin is built with it.
  • Configuration formats: grammars for structured formats like JSON. Grammar-Kit includes a JSON grammar as a test example.
  • Domain-specific languages: custom expression languages, query languages, and other DSLs. Grammar-Kit's own BNF grammar is defined in itself.
  • Query languages: specialized parsers like the Cypher graph database plugin.

The generated PSI tree integrates directly with the IntelliJ Platform, providing the foundation that powers syntax highlighting, code completion, navigation, and other IDE features. Grammar-Kit does not generate those features itself, but it produces the structural layer they depend on.

Key capabilities

Grammar-Kit covers the full cycle from grammar authoring through code generation and IDE integration.

Live Preview lets you test a grammar against sample text in real time, without generating code or compiling anything. Open it with Ctrl+Alt+P (Cmd+Alt+P on macOS), and a structure view shows the PSI tree as you edit the grammar. This is the fastest way to iterate on grammar rules. See Live Preview for details.

Expression parsing handles operator precedence automatically. You define expression rules in your grammar, and Grammar-Kit generates a compact Pratt-like parser that supports binary, prefix, postfix, and n-ary operators with correct priority and associativity. The entire expression hierarchy compiles down to just two generated methods. See Expression Parsing for details.

Error recovery uses two attributes, pin and recoverWhile, to keep parsing useful even when input is incomplete or contains unexpected tokens. The pin attribute commits the parser at a specific position so it reports errors without abandoning the current rule. The recoverWhile attribute skips unexpected tokens until a recovery predicate matches. See Error Recovery for details.

Generated parsers integrate directly with the IntelliJ Platform. They extend LightPsiParser, and the runtime base class (GeneratedParserUtilBase) ships with the IntelliJ Platform itself since version 12.1, so you do not need to bundle it. A Gradle plugin supports automated parser generation in CI builds.

The typical workflow is: create a .bnf file, prototype with Live Preview, generate parser and PSI classes (Ctrl+Shift+G / Cmd+Shift+G), generate a JFlex lexer, then implement ParserDefinition and register it in plugin.xml. The Quick Start Tutorial walks through this process step by step.

To continue, see the Features overview, install Grammar-Kit, or jump to the Quick Start Tutorial.