How to enforce Tag value pattern in Azure policy?

Question

I'd like to enforce tag value pattern "RJGVM-###" for a Tag which will be required for resource groups. I manage to make it required, but whenever I put in any value it still passes.

{
  "mode": "All",
  "policyRule": {
    "if": {
      "allOf": [
        {
          "field": "type",
          "equals": "Microsoft.Resources/subscriptions/resourceGroups"
        },
        {
          "not": {
            "field": "[concat('tags[',parameters('tagName'), ']')]",
            "exists": "true"
          }
        },
        {
          "value": "[resourceGroup().tags[parameters('tagName')]]",
          "notMatch": "RJGVM-###"
        }
      ]
    },
    "then": {
      "effect": "deny"
    }
  },
  "parameters": {
    "tagName": {
      "type": "String",
      "metadata": {
        "displayName": "Tag Name",
        "description": "Name of the tag, such as 'environment'"
      }
    }
  }
}

Answer 1

Please refer this example mentioned in the below link to ensure match pattern on tag value :

https://github.com/Azure/azure-policy/tree/master/samples/TextPatterns/enforce-tag-match-pattern