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Quick Start Tutorial

This tutorial walks through building a complete language plugin for IntelliJ IDEA using Grammar-Kit. You write a grammar for a simple calculator language, generate parser and lexer code, wire up the plugin classes, and verify that everything works. The result is a working plugin that parses .calc files with syntax highlighting and error recovery.

Before starting, install Grammar-Kit and set up an IntelliJ plugin development project. This tutorial assumes you are comfortable with Java development and have a basic understanding of what Grammar-Kit does.

Your First Grammar

Create a file called Calc.bnf in your project with the following grammar:

// Calc.bnf — A simple calculator language.
// Supports: numbers, +, -, *, /, parentheses, semicolons.
{
  // --- Package and class names for code generation ---
  parserClass="com.example.calc.parser.CalcParser"
  elementTypeHolderClass="com.example.calc.psi.CalcTypes"
  psiPackage="com.example.calc.psi"
  psiImplPackage="com.example.calc.psi.impl"
  psiVisitorName="CalcVisitor"

  // --- Token definitions ---
  tokens=[
    SEMI=';'
    PLUS='+'
    MINUS='-'
    MULT='*'
    DIV='/'
    LP='('
    RP=')'

    space='regexp:\s+'
    comment='regexp://.*'
    number='regexp:\d+(\.\d*)?'
  ]
}

// --- Rules ---
root ::= statement *
private statement ::= expr ';' {pin=1 recoverWhile=statement_recover}
private statement_recover ::= !(number | '(' | ';')

expr ::= add_expr
add_expr ::= mul_expr (('+'|'-') mul_expr) *
mul_expr ::= primary_expr (('*'|'/') primary_expr) *
primary_expr ::= number | paren_expr
paren_expr ::= '(' expr ')' {pin=1}

A .bnf file has two sections: a header attributes block in curly braces, followed by the grammar rules. The header controls code generation and defines tokens. The rules describe the language structure.

The header attributes tell Grammar-Kit where to put the generated code:

Attribute Value Purpose
parserClass com.example.calc.parser.CalcParser Generated parser class
elementTypeHolderClass com.example.calc.psi.CalcTypes Token and element type constants
psiPackage com.example.calc.psi PSI interface package
psiImplPackage com.example.calc.psi.impl PSI implementation package
psiVisitorName CalcVisitor Generated visitor class name

Always set these attributes explicitly. Without them, Grammar-Kit generates code into a generated package with default names, which makes the output harder to find and organize.

The tokens block defines the lexical elements of the language. Simple tokens like SEMI=';' match exact characters. Regexp tokens like number='regexp:\d+(\.\d*)?' use the regexp: prefix and match patterns. The space and comment regexp tokens are special: because no grammar rule references them, Grammar-Kit's Live Preview auto-detects them as whitespace and comments.

Token Definition Purpose
SEMI=';' Simple token Exact character match
number='regexp:\d+(\.\d*)?' Regexp token Matches 42, 3.14, 100.
space='regexp:\s+' Regexp token Auto-whitespace in Live Preview
comment='regexp://.*' Regexp token Auto-comment in Live Preview

The rules section defines how tokens combine into language constructs. The basic syntax elements are:

  • Sequence: rule ::= part1 part2 part3 matches parts in order
  • Choice: rule ::= part1 | part2 | part3 matches any alternative
  • Optional: [optional_part] or optional_part?
  • Repetition: part * (zero or more) or part + (one or more)
  • Grouping: (group_a | group_b) rest

Rules reference other rules by name (like mul_expr in the add_expr rule) and tokens by their quoted value (like '+') or their name (like SEMI). The private modifier on a rule means it does not create its own PSI node; instead, its children are included directly in the parent node.

The pin and recoverWhile attributes on the statement rule enable error recovery. When pin=1 is set, the parser commits to the rule after matching the first element. If subsequent elements fail, the parser reports an error but does not abandon the rule entirely. The recoverWhile attribute tells the parser to skip unexpected tokens until it sees something that could start a new statement (a number, (, or ;). These attributes are covered in depth in Error Recovery.

This grammar uses traditional precedence layers (add_expr calls mul_expr, which calls primary_expr) rather than Grammar-Kit's left/extends expression parsing idiom. That approach is more concise for complex expression languages and is covered in Expression Parsing.

Testing with Live Preview

Live Preview lets you test the grammar against sample input without generating any code. Open Calc.bnf in the editor and press Ctrl+Alt+P (Cmd+Alt+P on macOS). A split editor opens with the grammar on one side and a preview pane on the other. Type an expression like 1 + 2; in the preview pane, and Grammar-Kit parses it in real time using the grammar rules.

Two additional tools help during prototyping. The structure view (Ctrl+F12 / Cmd+F12) shows the PSI tree for the preview input. Grammar highlighting at caret (Ctrl+Alt+F7 / Cmd+Alt+F7 in the preview editor) highlights the grammar rules that match the current caret position in the preview.

Live Preview uses a simplified tokenizer built from the regexp tokens in the header block. This is sufficient for prototyping, but it does not match the behavior of a real JFlex lexer. Once your grammar stabilizes, switch to a generated JFlex lexer for accurate tokenization.

Generating Parser Code

With the grammar tested in Live Preview, generate the parser and PSI classes:

  1. Open Calc.bnf in the editor.
  2. Press Ctrl+Shift+G (Cmd+Shift+G on macOS), or right-click the file and select Generate Parser Code.
  3. A notification confirms success: "Calc.bnf generated (size)" with the output directory.

The generated files appear in the gen/ directory (configurable via the grammar.kit.gen.dir system property):

gen/
  com/example/calc/
    parser/
      CalcParser.java            ← Parser (static parse methods)
    psi/
      CalcTypes.java             ← IElementType constants
      CalcVisitor.java           ← PSI visitor
      CalcExpr.java              ← PSI interface (from expr rule)
      CalcAddExpr.java           ← PSI interface (from add_expr rule)
      CalcMulExpr.java           ← PSI interface (from mul_expr rule)
      CalcPrimaryExpr.java       ← PSI interface (from primary_expr rule)
      CalcParenExpr.java         ← PSI interface (from paren_expr rule)
      impl/
        CalcExprImpl.java        ← PSI implementation
        CalcAddExprImpl.java     ← PSI implementation
        CalcMulExprImpl.java     ← PSI implementation
        CalcPrimaryExprImpl.java ← PSI implementation
        CalcParenExprImpl.java   ← PSI implementation

Grammar-Kit generates one PSI interface and one implementation class for each public rule. Private rules like statement and statement_recover produce no PSI classes.

CalcTypes.java contains the element type and token type constants that the parser, lexer, and ParserDefinition all reference:

// Generated by Grammar-Kit — do not edit.
package com.example.calc.psi;

import com.intellij.psi.tree.IElementType;
import com.intellij.lang.ASTNode;
import com.intellij.psi.PsiElement;
import com.example.calc.psi.impl.*;

public interface CalcTypes {
  // Element types (one per public rule)
  IElementType ADD_EXPR = new CalcElementType("ADD_EXPR");
  IElementType EXPR = new CalcElementType("EXPR");
  IElementType MUL_EXPR = new CalcElementType("MUL_EXPR");
  IElementType PAREN_EXPR = new CalcElementType("PAREN_EXPR");
  IElementType PRIMARY_EXPR = new CalcElementType("PRIMARY_EXPR");

  // Token types (one per named token)
  IElementType DIV = new CalcTokenType("DIV");
  IElementType LP = new CalcTokenType("LP");
  IElementType MINUS = new CalcTokenType("MINUS");
  IElementType MULT = new CalcTokenType("MULT");
  IElementType NUMBER = new CalcTokenType("NUMBER");
  IElementType PLUS = new CalcTokenType("PLUS");
  IElementType RP = new CalcTokenType("RP");
  IElementType SEMI = new CalcTokenType("SEMI");

  // Factory method (creates PSI elements from AST nodes)
  // ...
}

CalcParser.java contains a static method for each grammar rule. Each method takes a PsiBuilder (the token stream interface) and an int recursion level (a guard against infinite recursion). Sub-expressions within a rule get suffixed names like add_expr_0. Avoid naming your own rules with this pattern (such as expr_0) because it conflicts with the generated names.

// Generated by Grammar-Kit — do not edit.
package com.example.calc.parser;

import com.intellij.lang.PsiBuilder;
import com.intellij.lang.PsiParser;
// ...

public class CalcParser implements PsiParser {
  // One static method per rule:
  // static boolean root(PsiBuilder b, int l) { ... }
  // static boolean expr(PsiBuilder b, int l) { ... }
  // static boolean add_expr(PsiBuilder b, int l) { ... }
  // static boolean mul_expr(PsiBuilder b, int l) { ... }
  // static boolean primary_expr(PsiBuilder b, int l) { ... }
  // static boolean paren_expr(PsiBuilder b, int l) { ... }
  //
  // Sub-expressions get suffixed names:
  // static boolean add_expr_0(PsiBuilder b, int l) { ... }
}

The generated PSI interfaces, implementations, and visitor class are covered in detail in the PSI customization section. For this tutorial, the defaults work without modification.

Note

Mark the gen/ directory as a Generated Sources Root in IntelliJ IDEA so the generated code compiles alongside your source code.

Creating a Language Plugin

The generated parser and PSI classes need a set of plugin classes to connect them to the IntelliJ Platform: a lexer, language and file type definitions, a parser definition, syntax highlighting, and plugin.xml registrations.

Generating the Lexer

Grammar-Kit generates a JFlex lexer definition from the token block in your grammar:

  1. Right-click Calc.bnf and select Generate JFlex Lexer. This creates _CalcLexer.flex.
  2. Right-click _CalcLexer.flex and select Run JFlex Generator. This creates _CalcLexer.java. (JFlex 1.9.2 is downloaded automatically if needed.)

The generated .flex file converts the regexp: token definitions from Java regex syntax to JFlex syntax (for example, \d becomes [0-9] and \s becomes [ \t\n\x0B\f\r]). It implements the FlexLexer interface and returns IElementType constants from CalcTypes:

package com.example.calc.parser;

import com.intellij.lexer.FlexLexer;
import com.intellij.psi.tree.IElementType;

import static com.intellij.psi.TokenType.BAD_CHARACTER;
import static com.intellij.psi.TokenType.WHITE_SPACE;
import static com.example.calc.psi.CalcTypes.*;

%%

%{
  public _CalcLexer() {
    this((java.io.Reader)null);
  }
%}

%public
%class _CalcLexer
%implements FlexLexer
%function advance
%type IElementType
%unicode

EOL=\R
WHITE_SPACE=\s+

NUMBER=[0-9]+(\.[0-9]*)?
COMMENT=//.*

%%
<YYINITIAL> {
  {WHITE_SPACE}    { return WHITE_SPACE; }

  ";"              { return SEMI; }
  "+"              { return PLUS; }
  "-"              { return MINUS; }
  "*"              { return MULT; }
  "/"              { return DIV; }
  "("              { return LP; }
  ")"              { return RP; }

  {NUMBER}         { return NUMBER; }
  {COMMENT}        { return COMMENT; }

}

[^] { return BAD_CHARACTER; }

The BAD_CHARACTER fallback at the end handles any input the lexer does not recognize. You can edit the .flex file manually for more complex lexing requirements.

Plugin Classes

The Calc plugin requires six hand-written Java classes. Each one is short and follows a standard pattern.

CalcLanguage.java defines the language singleton. The "Calc" string is the language ID, referenced by all other registrations:

package com.example.calc;

import com.intellij.lang.Language;

public class CalcLanguage extends Language {

    public static final CalcLanguage INSTANCE = new CalcLanguage();

    private CalcLanguage() {
        super("Calc");
    }
}

CalcFileType.java maps the .calc file extension to the Calc language:

package com.example.calc;

import com.intellij.openapi.fileTypes.LanguageFileType;
import org.jetbrains.annotations.NotNull;

import javax.swing.*;

public class CalcFileType extends LanguageFileType {

    public static final CalcFileType INSTANCE = new CalcFileType();

    private CalcFileType() {
        super(CalcLanguage.INSTANCE);
    }

    @Override
    public @NotNull String getName() {
        return "Calc";
    }

    @Override
    public @NotNull String getDescription() {
        return "Calc language file";
    }

    @Override
    public @NotNull String getDefaultExtension() {
        return "calc";
    }

    @Override
    public Icon getIcon() {
        return null; // Add a 16x16 icon later
    }
}

CalcFile.java connects the language and file type through a PsiFileBase subclass:

package com.example.calc;

import com.intellij.extapi.psi.PsiFileBase;
import com.intellij.openapi.fileTypes.FileType;
import com.intellij.psi.FileViewProvider;
import org.jetbrains.annotations.NotNull;

public class CalcFile extends PsiFileBase {

    public CalcFile(@NotNull FileViewProvider viewProvider) {
        super(viewProvider, CalcLanguage.INSTANCE);
    }

    @Override
    public @NotNull FileType getFileType() {
        return CalcFileType.INSTANCE;
    }
}

CalcParserDefinition.java is the central integration point. It tells the IntelliJ Platform how to create a lexer, parser, and PSI elements for Calc files:

package com.example.calc;

import com.intellij.lang.ASTNode;
import com.intellij.lang.ParserDefinition;
import com.intellij.lang.PsiParser;
import com.intellij.lexer.Lexer;
import com.intellij.openapi.project.Project;
import com.intellij.psi.FileViewProvider;
import com.intellij.psi.PsiElement;
import com.intellij.psi.PsiFile;
import com.intellij.psi.tree.IFileElementType;
import com.intellij.psi.tree.TokenSet;
import com.example.calc.parser.CalcParser;
import com.example.calc.parser._CalcLexer;
import com.example.calc.psi.CalcTypes;
import com.intellij.lexer.FlexAdapter;
import org.jetbrains.annotations.NotNull;

public class CalcParserDefinition implements ParserDefinition {

    public static final IFileElementType FILE =
        new IFileElementType("CALC_FILE", CalcLanguage.INSTANCE);

    @Override
    public @NotNull Lexer createLexer(Project project) {
        return new FlexAdapter(new _CalcLexer());
    }

    @Override
    public @NotNull PsiParser createParser(Project project) {
        return new CalcParser();
    }

    @Override
    public @NotNull IFileElementType getFileNodeType() {
        return FILE;
    }

    @Override
    public @NotNull TokenSet getWhitespaceTokens() {
        return TokenSet.WHITE_SPACE;
    }

    @Override
    public @NotNull TokenSet getCommentTokens() {
        return TokenSet.create(CalcTypes.COMMENT);
    }

    @Override
    public @NotNull TokenSet getStringLiteralElements() {
        return TokenSet.EMPTY;
    }

    @Override
    public @NotNull PsiElement createElement(ASTNode node) {
        return CalcTypes.Factory.createElement(node);
    }

    @Override
    public @NotNull PsiFile createFile(@NotNull FileViewProvider viewProvider) {
        return new CalcFile(viewProvider);
    }
}

The createLexer() method wraps the JFlex lexer in a FlexAdapter because JFlex generates a FlexLexer, not the Lexer type that the IntelliJ Platform expects. The createElement() method delegates to the generated factory in CalcTypes, which maps AST nodes to the correct PSI implementation classes.

CalcSyntaxHighlighter.java maps token types to editor colors using TextAttributesKey constants. Each key falls back to a standard color from DefaultLanguageHighlighterColors:

package com.example.calc;

import com.intellij.lexer.FlexAdapter;
import com.intellij.lexer.Lexer;
import com.intellij.openapi.editor.DefaultLanguageHighlighterColors;
import com.intellij.openapi.editor.colors.TextAttributesKey;
import com.intellij.openapi.fileTypes.SyntaxHighlighterBase;
import com.intellij.psi.TokenType;
import com.intellij.psi.tree.IElementType;
import com.example.calc.parser._CalcLexer;
import com.example.calc.psi.CalcTypes;
import org.jetbrains.annotations.NotNull;

import static com.intellij.openapi.editor.colors.TextAttributesKey.createTextAttributesKey;

public class CalcSyntaxHighlighter extends SyntaxHighlighterBase {

    public static final TextAttributesKey NUMBER =
        createTextAttributesKey("CALC_NUMBER",
            DefaultLanguageHighlighterColors.NUMBER);

    public static final TextAttributesKey COMMENT =
        createTextAttributesKey("CALC_COMMENT",
            DefaultLanguageHighlighterColors.LINE_COMMENT);

    public static final TextAttributesKey OPERATION =
        createTextAttributesKey("CALC_OPERATION",
            DefaultLanguageHighlighterColors.OPERATION_SIGN);

    public static final TextAttributesKey PARENTHESES =
        createTextAttributesKey("CALC_PARENTHESES",
            DefaultLanguageHighlighterColors.PARENTHESES);

    public static final TextAttributesKey BAD_CHAR =
        createTextAttributesKey("CALC_BAD_CHARACTER",
            DefaultLanguageHighlighterColors.INVALID_STRING_ESCAPE);

    @Override
    public @NotNull Lexer getHighlightingLexer() {
        return new FlexAdapter(new _CalcLexer());
    }

    @Override
    public TextAttributesKey @NotNull [] getTokenHighlights(IElementType type) {
        if (type.equals(CalcTypes.NUMBER)) {
            return pack(NUMBER);
        }
        if (type.equals(CalcTypes.COMMENT)) {
            return pack(COMMENT);
        }
        if (type.equals(CalcTypes.PLUS) || type.equals(CalcTypes.MINUS) ||
            type.equals(CalcTypes.MULT) || type.equals(CalcTypes.DIV)) {
            return pack(OPERATION);
        }
        if (type.equals(CalcTypes.LP) || type.equals(CalcTypes.RP)) {
            return pack(PARENTHESES);
        }
        if (type.equals(TokenType.BAD_CHARACTER)) {
            return pack(BAD_CHAR);
        }
        return TextAttributesKey.EMPTY_ARRAY;
    }
}

CalcSyntaxHighlighterFactory.java is a factory class required by the extension point:

package com.example.calc;

import com.intellij.openapi.fileTypes.SyntaxHighlighter;
import com.intellij.openapi.fileTypes.SyntaxHighlighterBase;
import com.intellij.openapi.fileTypes.SyntaxHighlighterFactory;
import com.intellij.openapi.project.Project;
import com.intellij.openapi.vfs.VirtualFile;
import org.jetbrains.annotations.NotNull;

public class CalcSyntaxHighlighterFactory extends SyntaxHighlighterFactory {

    @Override
    public @NotNull SyntaxHighlighter getSyntaxHighlighter(
            Project project, VirtualFile virtualFile) {
        return new CalcSyntaxHighlighter();
    }
}

Registering in plugin.xml

Register the file type, parser definition, and syntax highlighter in your plugin.xml:

<idea-plugin>
    <id>com.example.calc</id>
    <name>Calc Language</name>
    <vendor>Example</vendor>
    <description>Support for the Calc expression language.</description>

    <depends>com.intellij.modules.lang</depends>

    <extensions defaultExtensionNs="com.intellij">
        <!-- File type: maps .calc files to the Calc language -->
        <fileType name="Calc"
                  implementationClass="com.example.calc.CalcFileType"
                  fieldName="INSTANCE"
                  extensions="calc"
                  language="Calc"/>

        <!-- Parser: connects grammar to the IDE -->
        <lang.parserDefinition
                  language="Calc"
                  implementationClass="com.example.calc.CalcParserDefinition"/>

        <!-- Syntax highlighting -->
        <lang.syntaxHighlighterFactory
                  language="Calc"
                  implementationClass="com.example.calc.CalcSyntaxHighlighterFactory"/>
    </extensions>
</idea-plugin>

Three registrations are required at minimum: fileType, lang.parserDefinition, and lang.syntaxHighlighterFactory. The com.intellij.modules.lang dependency is also required for language support features.

Warning

The language="Calc" attribute in plugin.xml must match the string in the CalcLanguage constructor exactly. This comparison is case-sensitive: "Calc" and "calc" are different language IDs.

Project Layout

The complete file layout includes both hand-written and generated files:

my-calc-plugin/
  src/
    com/example/calc/
      CalcLanguage.java              ← You write this
      CalcFileType.java              ← You write this
      CalcFile.java                  ← You write this
      CalcParserDefinition.java      ← You write this
      CalcSyntaxHighlighter.java     ← You write this
      CalcSyntaxHighlighterFactory.java ← You write this
      parser/
        _CalcLexer.flex              ← Generated, then hand-edited
        _CalcLexer.java              ← Generated from .flex by JFlex
  gen/
    com/example/calc/
      parser/
        CalcParser.java              ← Generated from .bnf
      psi/
        CalcTypes.java               ← Generated from .bnf
        CalcVisitor.java             ← Generated from .bnf
        CalcExpr.java                ← Generated from .bnf
        CalcAddExpr.java             ← Generated from .bnf
        CalcMulExpr.java             ← Generated from .bnf
        CalcPrimaryExpr.java         ← Generated from .bnf
        CalcParenExpr.java           ← Generated from .bnf
        impl/
          CalcExprImpl.java          ← Generated from .bnf
          CalcAddExprImpl.java       ← Generated from .bnf
          CalcMulExprImpl.java       ← Generated from .bnf
          CalcPrimaryExprImpl.java   ← Generated from .bnf
          CalcParenExprImpl.java     ← Generated from .bnf
  resources/
    META-INF/
      plugin.xml                     ← You write this
  grammars/
    Calc.bnf                         ← You write this

Both src/ and gen/ must be on the classpath for the project to compile.

Testing Your Parser

Run the plugin using a standard IntelliJ plugin run configuration. In the sandbox IDE instance, create a file with a .calc extension and verify that syntax highlighting appears.

Try these sample inputs to exercise different parts of the grammar:

1 + 2;
3 * 4 + 5;
(10 - 3) * 2;

// Compute a result
(1 + 2) * (3 + 4);
100 / (5 - 3);
3.14 * 2;

To test error recovery, try input with a deliberate error:

1 + 2;
+ ;
3 * 4;

The second line (+ ;) has no left operand. The parser reports an error on that line but recovers at the semicolon and parses the third line successfully.

The PSI tree for a simple expression like 1 + 2; looks like this:

CalcFile
  Expr(ADD_EXPR)
    Expr(PRIMARY_EXPR)
      PsiElement(number)('1')
    PsiElement(PLUS)('+')
    Expr(PRIMARY_EXPR)
      PsiElement(number)('2')
  PsiElement(SEMI)(';')

Public rules become PSI nodes in the tree. Private rules (like statement) are transparent and do not appear. Tokens appear as PsiElement leaves.

Open Tools > View PSI Structure to inspect the parse tree for any file in your project. This is useful for verifying that the grammar produces the expected structure. You can also use the structure view (Ctrl+F12 / Cmd+F12) for a quick tree outline.

For automated regression testing, extend ParsingTestCase from the IntelliJ test framework. This base class lets you provide input text and compare the resulting PSI tree against an expected dump file. See the IntelliJ SDK Parsing Test documentation for setup details.

Grammar-Kit also provides built-in inspections in the BNF editor that catch common grammar problems: left recursion, unresolved references, unused rules, duplicate rules, suspicious tokens, and identical or unreachable choice branches. These inspections run as you edit the grammar and appear as warnings in the editor.

Common Mistakes

Without parserClass, elementTypeHolderClass, psiPackage, and psiImplPackage in the header, the generated code lands in a generated package with generic names. Always set these attributes explicitly.

A recovery predicate like !(number) skips the entire file if the input has no numbers. Include all possible statement starters and delimiters in the predicate: !(number | '(' | ';').

The generator creates methods like expr_0 for sub-expressions of expr. If you name your own rule expr_0, it conflicts with the generated name. Use descriptive names like primary_expr or literal instead.

JFlex generates a FlexLexer, not a Lexer. Wrap it with new FlexAdapter(new _CalcLexer()) in createLexer(). Similarly, the string in CalcLanguage's constructor ("Calc") must match language="Calc" in plugin.xml exactly, including case.

Workflow Summary

The full sequence from grammar to working plugin:

  1. Create Calc.bnf with tokens and rules.
  2. Open Live Preview (Ctrl+Alt+P) to test the grammar interactively.
  3. Generate parser code (Ctrl+Shift+G).
  4. Generate the JFlex lexer (right-click .bnf > Generate JFlex Lexer), then generate the Java lexer (right-click .flex > Run JFlex Generator).
  5. Write CalcLanguage, CalcFileType, CalcFile, and CalcParserDefinition.
  6. Write CalcSyntaxHighlighter and CalcSyntaxHighlighterFactory.
  7. Register everything in plugin.xml.
  8. Run the plugin and open a .calc file.

The recommended long-term workflow is: prototype the grammar in Live Preview, generate the initial .flex file and Java lexer, create the ParserDefinition and set up tests, then perfect the .flex and .bnf separately in the production environment.

For next steps, see BNF Grammar Syntax for the full syntax reference, Expression Parsing for the left/extends idiom, Error Recovery for advanced pin and recoverWhile patterns, or Gradle Integration for build automation.