diff --git a/extension.js b/extension.js
new file mode 100644
index 0000000..8805aa1
--- /dev/null
+++ b/extension.js
@@ -0,0 +1,181 @@
+const vscode = require('vscode');
+
+// regex patterns
+const SYMBOL_PREFIX = '^\\s*(?:[?!])?';
+const SYMBOL_SUFFIX = '(?:\\.\\d+)?';
+const TERM_BODY = '([A-Z_][A-Z_0-9]*)';
+const RULE_BODY = '([a-z_][a-z_0-9]*)';
+
+function activate(context) {
+    const selector = { language: 'lark', scheme: 'file' };
+    const provider = new LarkSymbolProvider();
+    context.subscriptions.push(vscode.languages.registerDocumentSymbolProvider(selector, provider));
+    // Diagnostics for unused or undefined symbols
+    const diagnosticCollection = vscode.languages.createDiagnosticCollection('lark-diagnostics');
+    context.subscriptions.push(diagnosticCollection);
+    // Validate currently open Lark document
+    if (vscode.window.activeTextEditor) {
+        const doc = vscode.window.activeTextEditor.document;
+        if (doc.languageId === 'lark') validateTextDocument(doc, diagnosticCollection);
+    }
+    // Re-validate on change, open, close
+    context.subscriptions.push(
+        vscode.workspace.onDidChangeTextDocument(e => validateTextDocument(e.document, diagnosticCollection))
+    );
+    context.subscriptions.push(
+        vscode.workspace.onDidOpenTextDocument(doc => validateTextDocument(doc, diagnosticCollection))
+    );
+    context.subscriptions.push(
+        vscode.workspace.onDidCloseTextDocument(doc => diagnosticCollection.delete(doc.uri))
+    );
+}
+
+class LarkSymbolProvider {
+    provideDocumentSymbols(document) {
+        const symbols = [];
+        const termRe = new RegExp(`${SYMBOL_PREFIX}${TERM_BODY}${SYMBOL_SUFFIX}\\s*`); // optional ?/! prefix, uppercase identifiers, optional .n suffix
+        const ruleRe = new RegExp(`${SYMBOL_PREFIX}${RULE_BODY}${SYMBOL_SUFFIX}\\s*`); // optional ?/! prefix, lowercase identifiers, optional .n suffix
+
+        for (let i = 0; i < document.lineCount; i++) {
+            const text = document.lineAt(i).text; // text content of the current line
+
+            function pushSymbol(match, kind) {
+                if (!match) return false;
+                const name = match[1];
+                const range = new vscode.Range(i, 0, i, text.length);
+                symbols.push(new vscode.DocumentSymbol(name, '', kind, range, range));
+                return true;
+            }
+
+            if (pushSymbol(termRe.exec(text), vscode.SymbolKind.Constant)) continue;
+            pushSymbol(ruleRe.exec(text), vscode.SymbolKind.Function);
+        }
+        return symbols;
+    }
+}
+
+// Check for unused grammar symbols and report warnings
+async function validateTextDocument(document, diagnosticCollection) {
+    if (document.languageId !== 'lark') return;
+
+    // Collect definitions via DocumentSymbolProvider
+    const symbols = await vscode.commands.executeCommand('vscode.executeDocumentSymbolProvider', document.uri) || [];
+    // Add imported terminals to symbols list
+    const importRe = /^\s*%import[^A-Z0-9_]*([A-Z0-9_]+)/;
+    for (let i = 0; i < document.lineCount; i++) {
+        const lineText = document.lineAt(i).text;
+        const importMatch = importRe.exec(lineText);
+        if (importMatch) {
+            const name = importMatch[1];
+            const kind = vscode.SymbolKind.Constant;
+            const range = new vscode.Range(i, 0, i, lineText.length);
+            symbols.push(new vscode.DocumentSymbol(name, '', kind, range, range));
+        }
+    }
+    /**
+     * Mapping of symbol names to their definition lines and usage status.
+     *
+     * Example: { 'MyTerminal': { line: 5, used: true } }
+     */
+    const defs = {};
+    (function flatten(list) {
+        for (const sym of list) {
+            defs[sym.name] = { line: sym.range.start.line, used: false };
+            if (sym.children && sym.children.length) flatten(sym.children);
+        }
+    })(symbols);
+    // Search for references
+    for (let i = 0; i < document.lineCount; i++) {
+        const text = document.lineAt(i).text;
+        for (const name in defs) {
+            if (defs[name].used || i === defs[name].line) continue;
+            if (new RegExp(`\\b${name}\\b`).test(text)) {
+                defs[name].used = true;
+            }
+        }
+    }
+
+    // Detect unused symbols and report warnings
+    const diagnostics = [];
+    for (const name in defs) {
+        if (!defs[name].used && name !== 'start') {
+            const lineNum = defs[name].line;
+            const line = document.lineAt(lineNum);
+            const startChar = line.text.indexOf(name);
+            if (startChar >= 0) {
+                const range = new vscode.Range(lineNum, startChar, lineNum, startChar + name.length);
+                diagnostics.push(new vscode.Diagnostic(range, `Unused grammar symbol '${name}'`, vscode.DiagnosticSeverity.Warning));
+            }
+        }
+    }
+
+    // Detect undefined symbols and report errors
+    const termRe = new RegExp(`${SYMBOL_PREFIX}${TERM_BODY}${SYMBOL_SUFFIX}\\s*:`);
+    const ruleRe = new RegExp(`${SYMBOL_PREFIX}${RULE_BODY}${SYMBOL_SUFFIX}\\s*:`);
+    for (let i = 0; i < document.lineCount; i++) {
+        const text = document.lineAt(i).text;
+        // skip directive lines and comments
+        if (text.trim().startsWith('%')) continue;
+        if (text.trim().startsWith('//')) continue;
+
+        let searchText = text;
+        const termDefMatch = termRe.exec(text);
+        const ruleDefMatch = ruleRe.exec(text);
+        const defHeadMatch = /^\s*([^:\s]+)\s*:/.exec(text); // match text before ':'
+        let offset = 0;
+        if (termDefMatch || ruleDefMatch) {
+            const colonIndex = text.indexOf(':');
+            if (colonIndex >= 0) {
+                offset = colonIndex + 1;
+                searchText = text.slice(offset);
+            }
+        } else if (defHeadMatch) {
+            // Report error for invalid definition head before ':'
+            const head = defHeadMatch[1];
+            const start = text.indexOf(head);
+            const end = start + head.length;
+            const range = new vscode.Range(i, start, i, end);
+            diagnostics.push(new vscode.Diagnostic(range, `Invalid definition name '${head}'`, vscode.DiagnosticSeverity.Error));
+        }
+
+        // strip literal strings in quotes
+        searchText = searchText.replace(/"[^"]*"/g, (match) => ' '.repeat(match.length));
+        // strip comments and aliases
+        searchText = searchText.split(/\/\/|->/)[0];
+        // strip regex
+        searchText = searchText.replace(/(\/(?:\\.|[^\/\\])*\/[gimsuy]*)/g, (match) => ' '.repeat(match.length));
+
+        /**
+         * Search for occurrences of a given regex pattern in the current line's text content,
+         * and report errors for any undefined symbols found.
+         *
+         * @param {RegExp} usageRe - Regex pattern to search for. It should have one capture group
+         *   that matches the symbol name.
+         * @param {string} type - Whether the symbol is a terminal or rule.
+         */
+        function checkUsages(usageRe, type) {
+            let usageMatch;
+            while ((usageMatch = usageRe.exec(searchText)) !== null) {
+                const name = usageMatch[1];
+                if (type === 'rule' && name === 'start') {
+                    continue;
+                }
+                if (!defs.hasOwnProperty(name)) {
+                    const realStart = offset + usageMatch.index;
+                    const range = new vscode.Range(i, realStart, i, realStart + name.length);
+                    const message = type === 'terminal'
+                        ? `Undefined terminal '${name}'`
+                        : `Undefined rule '${name}'`;
+                    diagnostics.push(new vscode.Diagnostic(range, message, vscode.DiagnosticSeverity.Error));
+                }
+            }
+        }
+        checkUsages(new RegExp(`\\b${TERM_BODY}${SYMBOL_SUFFIX}\\b`, 'g'), 'terminal');
+        checkUsages(new RegExp(`\\b${RULE_BODY}${SYMBOL_SUFFIX}\\b`, 'g'), 'rule');
+    }
+    diagnosticCollection.set(document.uri, diagnostics);
+}
+
+function deactivate() { }
+
+module.exports = { activate, deactivate };
diff --git a/package.json b/package.json
index 93086f9..d670d62 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
     "name": "vscode-lark",
-    "version": "0.1.0",
+    "version": "0.2.1",
     "publisher": "dirk-thomas",
     "engines": {
         "vscode": "^1.2.0"
@@ -19,6 +19,10 @@
     "bugs": {
         "url": "https://github.com/dirk-thomas/vscode-lark/issues"
     },
+    "main": "./extension.js",
+    "activationEvents": [
+        "onLanguage:lark"
+    ],
     "contributes": {
         "languages": [
             {
diff --git a/syntaxes/lark.json b/syntaxes/lark.json
index 3672310..e4498b9 100644
--- a/syntaxes/lark.json
+++ b/syntaxes/lark.json
@@ -78,4 +78,4 @@
         }
     ],
     "scopeName": "source.lark"
-}
+}
\ No newline at end of file