Regex filters

Match and redact prompt content with custom regex patterns or agentgateway’s built-in PII detectors.

Use custom regex patterns and built-in PII detectors to filter LLM requests and responses.

About regex prompt templating

Regex-based prompt guards let you inspect LLM requests and responses against custom regex patterns or built-in PII detectors. Use the reject action to block requests that match a pattern, or the mask action to redact sensitive data in responses before they reach the client.

Built-in prompt guard patterns

Agentgateway includes the following built-in patterns for common PII types that you can reference in your prompt guards.

Pattern	Description
`email`	Email addresses
`phoneNumber`	Phone numbers
`ssn`	Social Security Numbers
`creditCard`	Credit card numbers
`caSin`	Canadian Social Insurance Numbers

Custom regex patterns

Use custom patterns to match credentials, secrets, or application-specific sensitive data.

llm:
  models:
  - name: "*"
    provider: openAI
    params:
      model: gpt-4o-mini
      apiKey: "$OPENAI_API_KEY"
    guardrails:
      request:
      - regex:
          action: reject
          rules:
          - pattern: "password[=:]\\s*\\S+"
          - pattern: "api[_-]?key[=:]\\s*\\S+"
          - pattern: "secret[=:]\\s*\\S+"
        rejection:
          status: 400
          headers:
            set:
              content-type: "application/json"
          body: |
            {
              "error": {
                "message": "Request contains credentials",
                "type": "invalid_request_error",
                "code": "credentials_detected"
              }
            }

Before you begin

Install the agentgateway binary.

PII detection

The following example rejects requests that contain PII data, such as Social Security Numbers (using a custom keyword pattern) or email addresses (using the built-in email pattern). When a request is blocked, agentgateway returns a custom error response.

Create a configuration file with regex prompt guard policies.

cat <<'EOF' > config.yaml
# yaml-language-server: $schema=https://agentgateway.dev/schema/config
llm:
  models:
  - name: "*"
    provider: openAI
    params:
      model: gpt-4o-mini
      apiKey: "$OPENAI_API_KEY"
    guardrails:
      request:
      - regex:
          action: reject
          rules:
          - pattern: SSN
          - pattern: Social Security
        rejection:
          status: 400
          headers:
            set:
              content-type: "application/json"
          body: |
            {
              "error": {
                "message": "Request rejected: Content contains sensitive information",
                "type": "invalid_request_error",
                "code": "content_policy_violation"
              }
            }
      - regex:
          action: reject
          rules:
          - builtin: email
        rejection:
          status: 400
          headers:
            set:
              content-type: "application/json"
          body: |
            {
              "error": {
                "message": "Request blocked: Contains email address",
                "type": "invalid_request_error",
                "code": "pii_detected"
              }
            }
EOF

Setting	Description
`regex.action`	The action to take when a pattern matches. Use `reject` to block the request or `mask` to redact matched content.
`regex.rules`	List of patterns to match against.
`pattern`	A custom regex pattern.
`builtin`	A built-in PII pattern. See Built-in patterns for available options.
`rejection`	Custom response returned when a request is blocked. Specify an HTTP `status` code, response `headers`, and a `body`.

Start the agentgateway.
```
agentgateway -f config.yaml
```

In a new terminal, send a request to your LLM provider. Verify that the request succeeds.

curl http://localhost:4000/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
    "model": "gpt-4o-mini",
    "messages": [{"role": "user", "content": "Hello, how are you?"}]
  }'

Example output:

:0},"prompt_tokens_details":{"cached_tokens":0,
"audio_tokens":0}},"choices":[{"message":
{"content":"Hello! I'm just a program, but I'm here and 
ready to help you. How can I assist you today?",
"role":"assistant","refusal":null,"annotations":[]},
"index":0,"logprobs":null,"finish_reason":"stop"}],
"id":"chatcmpl-DHwlvtADPu5ZFznynSpmSjXL4B6W3",
"object":"chat.completion",
"service_tier":"default",
"system_fingerprint":"fp_a1ddba3226"}

Send a request containing the SSN keyword. The prompt guard blocks the request and returns your custom error response.

curl http://localhost:4000/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
    "model": "gpt-4o-mini",
    "messages": [{"role": "user", "content": "My SSN is 123-45-6789"}]
  }'

Example output:

{
  "error": {
    "message": "Request rejected: Content contains sensitive information",
    "type": "invalid_request_error",
    "code": "content_policy_violation"
  }
}

Send another request with an email address. The prompt guard blocks it by using the built-in email pattern.

curl http://localhost:4000/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
    "model": "gpt-4o-mini",
    "messages": [{"role": "user", "content": "Contact me at test@example.com"}]
  }'

Example output:

{
  "error": {
    "message": "Request blocked: Contains email address",
    "type": "invalid_request_error",
    "code": "pii_detected"
  }
}

Mask PII in responses

You can also filter LLM responses to redact sensitive data before it reaches the client. When a match is found, agentgateway replaces built-in pattern matches with <ENTITY_TYPE> (for example, <CREDIT_CARD>) and custom pattern matches with <masked>. The following example masks credit card numbers in responses.

Create a configuration that masks phone numbers in LLM responses by using the built-in phoneNumber pattern.

cat <<'EOF' > config.yaml
# yaml-language-server: $schema=https://agentgateway.dev/schema/config
llm:
  models:
  - name: "*"
    provider: openAI
    params:
      model: gpt-4o-mini
      apiKey: "$OPENAI_API_KEY"
    guardrails:
      response:
      - regex:
          action: mask
          rules:
          - builtin: phoneNumber
EOF

Start the agentgateway.
```
agentgateway -f config.yaml
```

In a new terminal, send a request to your LLM provider with a phone number and verify that the number is masked in your response.

curl http://localhost:4000/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
    "model": "gpt-4o-mini",
    "messages": [{"role": "user", "content": "What number is 919 222 1111?"}]
  }'

Example output:

{"model":"gpt-4o-mini-2024-07-18","usage":
{"prompt_tokens":18,"completion_tokens":57,
"total_tokens":75,"completion_tokens_details":
{"reasoning_tokens":0,"audio_tokens":0,
"accepted_prediction_tokens":0,
"rejected_prediction_tokens":0},"prompt_tokens_details":
{"cached_tokens":0,"audio_tokens":0}},"choices":
[{"message":{"content":"The number <PHONE_NUMBER>appears 
to be a phone number in the United States. The area code
919 serves parts of North Carolina, including cities 
like Raleigh and Durham. If you have a specific 
question or need more information regarding this 
number, please let me know!","role":"assistant",
"refusal":null,"annotations":[]},"index":0,
"logprobs":null,"finish_reason":"stop"}],
"id":"chatcmpl-DHxEv3O5VOQPCmIVPruRiToal0rIe","object":"chat.completion","created":1773171665,
"service_tier":"default",
"system_fingerprint":"fp_a1ddba3226"}%

About guardrails OpenAI moderation

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Regex filters

About regex prompt templating

Built-in prompt guard patterns

Custom regex patterns

Before you begin

PII detection

Mask PII in responses

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