Update chat agents' output parser to extract action by regex (#7511)

Currently `ChatOutputParser` extracts actions by splitting the text on
"```", and then load the second part as a json string.

But sometimes the LLM will wrap the action in markdown code block like:

````markdown
```json
{
  "action": "foo",
  "action_input": "bar"
}
```
````

Splitting text on "```" will cause `OutputParserException` in such case.

This PR changes the behaviour to extract the `$JSON_BLOB` by regex, so
that it can handle both ` ``` ``` ` and ` ```json ``` `

@hinthornw

---------

Co-authored-by: Junlin Zhou <jlzhou@zjuici.com>

langchain/agents/chat/output_parser.py

@@ -1,4 +1,5 @@
 import json
+import re
 from typing import Union
 
 from langchain.agents.agent import AgentOutputParser
@@ -9,13 +10,19 @@ FINAL_ANSWER_ACTION = "Final Answer:"
 
 
 class ChatOutputParser(AgentOutputParser):
+    pattern = re.compile(r"^.*?`{3}(?:json)?\n(.*?)`{3}.*?$", re.DOTALL)
+
     def get_format_instructions(self) -> str:
         return FORMAT_INSTRUCTIONS
 
     def parse(self, text: str) -> Union[AgentAction, AgentFinish]:
         includes_answer = FINAL_ANSWER_ACTION in text
         try:
-            action = text.split("```")[1]
+            found = self.pattern.search(text)
+            if not found:
+                # Fast fail to parse Final Answer.
+                raise ValueError("action not found")
+            action = found.group(1)
             response = json.loads(action.strip())
             includes_action = "action" in response
             if includes_answer and includes_action:

tests/unit_tests/agents/test_chat.py

@@ -0,0 +1,47 @@
+"""Unittests for langchain.agents.chat package."""
+from typing import Tuple
+
+from langchain.agents.chat.output_parser import ChatOutputParser
+from langchain.schema import AgentAction
+
+output_parser = ChatOutputParser()
+
+
+def get_action_and_input(text: str) -> Tuple[str, str]:
+    output = output_parser.parse(text)
+    if isinstance(output, AgentAction):
+        return output.tool, str(output.tool_input)
+    else:
+        return "Final Answer", output.return_values["output"]
+
+
+def test_parse_with_language() -> None:
+    llm_output = """I can use the `foo` tool to achieve the goal.
+
+    Action:
+    ```json
+    {
+      "action": "foo",
+      "action_input": "bar"
+    }
+    ```
+    """
+    action, action_input = get_action_and_input(llm_output)
+    assert action == "foo"
+    assert action_input == "bar"
+
+
+def test_parse_without_language() -> None:
+    llm_output = """I can use the `foo` tool to achieve the goal.
+
+    Action:
+    ```
+    {
+      "action": "foo",
+      "action_input": "bar"
+    }
+    ```
+    """
+    action, action_input = get_action_and_input(llm_output)
+    assert action == "foo"
+    assert action_input == "bar"