customize length of the password in dialogflow chatbot using regex entity

Question

length of the password must be at least 6 characters while signing up using Dialogflow chatbot. I have used system entity "any". But it is so generalized. Webhook is written in flask python.

Also tried to do it by using [0-9]{6}$ as a regex entity. But unable to find a proper way to perform this task.

from dialogflow_fulfillment import QuickReplies, WebhookClient, Payload
from flask import Flask, request, Response, jsonify , make_response
import json
import requests

app = Flask(__name__)

def handler(agent: WebhookClient) :
    """Handle the webhook request.."""

    req = request.get_json(force=True)
    intent_name = req.get('queryResult').get('intent').get('displayName')

    if intent_name == 'intro':
        agent.add('I am the chatbot of this page. Ready to assist you with anything you need. What would you like to do?')
        agent.add(QuickReplies(quick_replies=['START NOW','LOGIN']))

    if intent_name == 'get_started':
        url = 'http://**********.com/create_account'
        userid = req.get('queryResult').get('parameters').get('email')
        print(userid)
        pwd = req.get('queryResult').get('parameters').get('pwd')
        print(pwd)
        name = req.get('queryResult').get('parameters').get('person')['name']
        print(name)
        age = req.get('queryResult').get('parameters').get('age')
        print(age)     
        myobj = {'userid': userid, 'pwd': pwd , 'name' : name, 'age' : age}
        x = requests.post(url, data = myobj)
        result=x.text
        agent.add(result)
        

    if intent_name == 'login_screen' :
        url = 'http://**********.com/auth_account'
        userid = req.get('queryResult').get('parameters').get('email')
        print(userid)
        pwd = req.get('queryResult').get('parameters').get('pwd')
        print(pwd)
        myobj = {'userid': userid, 'pwd': pwd }
        x = requests.post(url, data = myobj)
        result = x.text
        agent.add(result)

@app.route('/webhook', methods=['GET', 'POST'])
def webhook():
    """Handle webhook requests from Dialogflow."""
    req = request.get_json(force=True)
    agent = WebhookClient(req)
    agent.handle_request(handler)
    return agent.response

if __name__ == '__main__':
    app.run(debug=True)

With regex : Defined regex :

How to do this properly? How to use regex in it? Or should i use another approach?