[Docs] Add EBNF to sampling params docs (#2609)

docs/backend/openai_api_completions.ipynb

@@ -220,14 +220,21 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "## Structured decoding (JSON, Regex)\n",
-    "You can define a JSON schema or regular expression to constrain the model's output. The model output will be guaranteed to follow the given constraints and this depends on the grammar backend.\n",
+    "## Structured Outputs (JSON, Regex, EBNF)\n",
+    "You can specify a JSON schema, Regular Expression or [EBNF](https://en.wikipedia.org/wiki/Extended_Backus%E2%80%93Naur_form) to constrain the model output. The model output will be guaranteed to follow the given constraints. \n",
     "\n",
-    "SGlang has two backends: [Outlines](https://github.com/dottxt-ai/outlines) (default) and [XGrammar](https://blog.mlc.ai/2024/11/22/achieving-efficient-flexible-portable-structured-generation-with-xgrammar). Xgrammar accelerates JSON decoding performance but does not support regular expressions. To use Xgrammar, add the `--grammar-backend xgrammar` when launching the server:\n",
+    "SGLang supports two grammar backends:\n",
     "\n",
+    "- [Outlines](https://github.com/dottxt-ai/outlines) (default): Supports JSON schema and Regular Expression constraints.\n",
+    "- [XGrammar](https://github.com/mlc-ai/xgrammar): Supports JSON schema and EBNF constraints.\n",
+    "  - XGrammar currently uses the [GGML BNF format](https://github.com/ggerganov/llama.cpp/blob/master/grammars/README.md)\n",
+    "\n",
+    "> 🔔 Only one constraint parameter (`json_schema`, `regex`, or `ebnf`) can be specified at a time.\n",
+    "\n",
+    "Initialise xgrammar backend using `--grammar-backend xgrammar` flag\n",
     "```bash\n",
     "python -m sglang.launch_server --model-path meta-llama/Meta-Llama-3.1-8B-Instruct \\\n",
-    "--port 30000 --host 0.0.0.0 --grammar-backend xgrammar\n",
+    "--port 30000 --host 0.0.0.0 --grammar-backend [xgrammar|outlines] # xgrammar or outlines (default: outlines)\n",
     "```\n",
     "\n",
     "### JSON"
@@ -275,7 +282,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "### Regular expression"
+    "### Regular expression (use default \"outlines\" backend)"
    ]
   },
   {
@@ -297,6 +304,46 @@
     "print_highlight(response.choices[0].message.content)"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "### EBNF (use \"xgrammar\" backend)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "# terminate the existing server(that's using default outlines backend) for this demo\n",
+    "terminate_process(server_process)\n",
+    "\n",
+    "# start new server with xgrammar backend\n",
+    "server_process = execute_shell_command(\n",
+    "    \"python -m sglang.launch_server --model-path meta-llama/Meta-Llama-3.1-8B-Instruct --port 30000 --host 0.0.0.0 --grammar-backend xgrammar\"\n",
+    ")\n",
+    "wait_for_server(\"http://localhost:30000\")\n",
+    "\n",
+    "# EBNF example\n",
+    "ebnf_grammar = r\"\"\"\n",
+    "        root ::= \"Hello\" | \"Hi\" | \"Hey\"\n",
+    "        \"\"\"\n",
+    "response = client.chat.completions.create(\n",
+    "    model=\"meta-llama/Meta-Llama-3.1-8B-Instruct\",\n",
+    "    messages=[\n",
+    "        {\"role\": \"system\", \"content\": \"You are a helpful EBNF test bot.\"},\n",
+    "        {\"role\": \"user\", \"content\": \"Say a greeting.\"},\n",
+    "    ],\n",
+    "    temperature=0,\n",
+    "    max_tokens=32,\n",
+    "    extra_body={\"ebnf\": ebnf_grammar},\n",
+    ")\n",
+    "\n",
+    "print_highlight(response.choices[0].message.content)"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},