# Matching events on null values and empty strings in Amazon EventBridge
Matching on null values and empty strings

**Important**  
In EventBridge, it is possible to create rules that can lead to higher-than-expected charges and throttling. For example, you can inadvertently create a rule that leads to an infinite loop, where a rule is fired recursively without end. Suppose you created a rule to detect that ACLs have changed on an Amazon S3 bucket, and trigger software to change them to the desired state. If the rule is not written carefully, the subsequent change to the ACLs fires the rule again, creating an infinite loop.  
For guidance on how to write precise rules and event patterns to minimize such unexpected results, see [Best practices for rules](eb-rules-best-practices.md) and [Best practices](eb-patterns-best-practices.md).

You can create an [event pattern](eb-event-patterns.md) that matches a field in an [event](eb-events.md) that has a null value or is an empty string. Consider the following example event.

See best practices to avoid higher than expected charges and throttling

```
{
  "version": "0",
  "id": "3e3c153a-8339-4e30-8c35-687ebef853fe",
  "detail-type": "EC2 Instance Launch Successful",
  "source": "aws.autoscaling",
  "account": "123456789012",
  "time": "2015-11-11T21:31:47Z",
  "region": "us-east-1",
  "resources": [
   ],
  "detail": {
    "eventVersion": "",
    "responseElements": null
   }
}
```

To match events where the value of `eventVersion` is an empty string, use the following event pattern, which matches the preceding event.

```
{
  "detail": {
     "eventVersion": [""]
  }
}
```

To match events where the value of `responseElements` is null, use the following event pattern, which matches the preceding event.

```
{
  "detail": {
     "responseElements": [null]
  }
}
```

**Note**  
Null values and empty strings are not interchangeable in pattern matching. An event pattern that matches empty strings doesn't match values of `null`.

## Using null values in AWS CloudFormation templates


AWS CloudFormation does not allow `null` values in templates. If you define an event pattern with a null value using YAML or JSON object syntax, the template validation fails with the error: `'null' values are not allowed in templates`.

To work around this limitation, specify the `EventPattern` property as a JSON string instead of a YAML or JSON object. The following example shows how to match on null values in a AWS CloudFormation template:

```
MyRule:
  Type: AWS::Events::Rule
  Properties:
    EventPattern: '{"detail":{"responseElements":[null]}}'
```