Azure Policy to allow only lowercase string - azure

Question

I am planning to achieve the below naming pattern in Azure using a policy.
tstpaas-appName-Env-Serial
tstpaas is a string constant
appname can be string with length of 3-5 characters
env - dev/ust/prd
serial - three numbers
Example:
tstpaas-app-dev-001
All of the characters should be lower case. I am able to match the pattern, however, allowing 3-5 characters for app name and allowing only lower case values is not working. Please let me know how can I achieve this.
Here is my policy:
{
"mode": "All",
"policyRule": {
"if": {
"allOf": [
{
"field": "type",
"in": "[parameters('allowedResourceTypes')]"
},
{
"anyOf": [
{
"not": {
"field": "name",
"match": "[parameters('nameMatchPattern1')]"
}
},
{
"not": {
"value": "[if(greaterOrEquals(length(field('name')), 18), substring(field('name'), 14, 3), 'not in allowed values')]",
"in": "[parameters('allowedEnvironments')]"
}
}
]
}
]
},
"then": {
"effect": "deny"
}
},
"parameters": {
"nameMatchPattern1": {
"type": "String",
"metadata": {
"displayName": "nameMatchPattern1",
"description": "Pattern to match for names. Cna include ? and # for digits and letters"
}
},
"allowedResourceTypes": {
"type": "Array",
"metadata": {
"displayName": "allowedResourceTypes",
"description": "List of resource types, for which policy applies"
}
},
"allowedEnvironments": {
"type": "Array",
"metadata": {
"displayName": "Allowed Environments",
"description": "The list of Environments"
},
"allowedValues": [
"dev",
"int",
"uat",
"prd"
]
}
}
}
Parameter for namematchpatter is:
tstpaas-?????-???-###