---
title: Expression validation
description: Validate firewall rule expressions via the API.
image: https://developers.cloudflare.com/core-services-preview.png
---

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# Expression validation

The Cloudflare Filters API supports an endpoint for validating expressions.

## Examples

### Validate expression via query string

Request

```
curl "https://api.cloudflare.com/client/v4/filters/validate-expr?expression=ip.src==34" \--header "X-Auth-Email: <EMAIL>" \--header "X-Auth-Key: <API_KEY>"
```

Response

```
{  "result": null,  "success": false,  "errors": [    {      "message": "Filter parsing error:\n`ip.src==34`\n          ^^ couldn't parse address in network: invalid IP address syntax\n"    }  ],  "messages": null}
```

Note the validation error in the response. In this example, the error is due to an invalid IP address format:

```
Filter parsing error:`ip.src==34`          ^^ couldn't parse address in network: invalid IP address syntax
```

### Validate expression via JSON object

Request

```
curl "https://api.cloudflare.com/client/v4/filters/validate-expr" \--header "X-Auth-Email: <EMAIL>" \--header "X-Auth-Key: <API_KEY>" \--header "Content-Type: application/json" \--data '{  "expression": "ip.src in {2400:cb00::/32 2405:8100::/2000 2c0f:f248::/32 2a06:98c0::/29}"}'
```

Response

```
{  "result": null,  "success": false,  "errors": [    {      "message": "Filter parsing error:\n`ip.src in {2400:cb00::/32 2405:8100::/2000 2c0f:f248::/32 2a06:98c0::/29}`\n                                        ^^^^ number too large to fit in target type while parsing with radix 10\n"    }  ],  "messages": null}
```

Note the validation error in the response. In this example, the value for the subnet mask, `/2000`, is not a valid IPv6 CIDR mask:

```
Filter parsing error:`ip.src in {2400:cb00::/32 2405:8100::/2000 2c0f:f248::/32 2a06:98c0::/29}`                                       ^^^^ number too large to fit in target type while parsing with radix 10
```

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