Add a regex expression validation for required fields.

Sometimes LLM will hallucinate and simulate a value that does not exist. For example, it fills in a false value in the order number below. I want a user-defined validator that can validate the data format in advance.

{ "function": "route_to_agent",
  "reason": "User wants to pay in cash",
  "next_action_agent": "Payment",
  "user_goal_agent": "Payment",
  "args": {
    "order_number": "order number"
  }
}

Below is part of the Router prompt:

* Agent: Payment
Make payment when user wants to pay for the order
Required args: {
    "order_number": "order number",
    }

Start by tracing how the Router handles required args and agent prompts. Determine where a user-defined regex validator would fit, then verify that a fabricated order_number such as "order number" is rejected while a matching value is accepted.