Verify that new data starts with a certain string in Firebase

To create a Firebase Realtime Database rule to validate that new data starts with a certain string, for example with foo:

terminal
{
  "rules": {
    "concurrent": {
      ".read": true,
      ".write": true,
      "$uid": {
        "connected": {
          ".validate": "newData.val().matches(/^foo/)"
        }
      }
    }
  }
}

You can use RegEx within the matches() method.