Fix regex docs (#1909)

docs/backend/openai_api_completions.ipynb

@@ -87,7 +87,6 @@
     "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 AI assistant\"},\n",
     "        {\"role\": \"user\", \"content\": \"List 3 countries and their capitals.\"},\n",
     "    ],\n",
     "    temperature=0,\n",
@@ -184,7 +183,6 @@
     "## Completions\n",
     "\n",
     "### Usage\n",
-    "\n",
     "Completions API is similar to Chat Completions API, but without the `messages` parameter or chat templates."
    ]
   },
@@ -253,6 +251,77 @@
     "print_highlight(f\"Response: {response}\")"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Structured decoding (JSON, Regex)\n",
+    "You can specify a JSON schema or a regular expression to constrain the model output. The model output will be guaranteed to follow the given constraints.\n",
+    "\n",
+    "### JSON"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "import json\n",
+    "\n",
+    "json_schema = json.dumps(\n",
+    "    {\n",
+    "        \"type\": \"object\",\n",
+    "        \"properties\": {\n",
+    "            \"name\": {\"type\": \"string\", \"pattern\": \"^[\\\\w]+$\"},\n",
+    "            \"population\": {\"type\": \"integer\"},\n",
+    "        },\n",
+    "        \"required\": [\"name\", \"population\"],\n",
+    "    }\n",
+    ")\n",
+    "\n",
+    "response = client.chat.completions.create(\n",
+    "    model=\"meta-llama/Meta-Llama-3.1-8B-Instruct\",\n",
+    "    messages=[\n",
+    "        {\"role\": \"user\", \"content\": \"Give me the information of the capital of France in the JSON format.\"},\n",
+    "    ],\n",
+    "    temperature=0,\n",
+    "    max_tokens=128,\n",
+    "    response_format={\n",
+    "        \"type\": \"json_schema\",\n",
+    "        \"json_schema\": {\"name\": \"foo\", \"schema\": json.loads(json_schema)},\n",
+    "    },\n",
+    ")\n",
+    "\n",
+    "print_highlight(response.choices[0].message.content)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "### Regular expression"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "response = client.chat.completions.create(\n",
+    "    model=\"meta-llama/Meta-Llama-3.1-8B-Instruct\",\n",
+    "    messages=[\n",
+    "        {\"role\": \"user\", \"content\": \"What is the capital of France?\"},\n",
+    "    ],\n",
+    "    temperature=0,\n",
+    "    max_tokens=128,\n",
+    "    extra_body={\"regex\": \"(Paris|London)\"},\n",
+    ")\n",
+    "\n",
+    "print_highlight(response.choices[0].message.content)"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},