Custom Regex

Create in-process regex guardrails for organization-specific policies, including the built-in PII Detection template.

Overview

Custom Regex is a Bifrost Enterprise guardrail provider that evaluates request and response text against regex patterns you define.

It is useful when you need a simple, local policy check without calling an external guardrail provider. Common uses include blocking organization-specific IDs, internal project names, environment-specific secrets, and PII-like text patterns.

Custom Regex runs in-process and uses Go’s RE2-compatible regexp engine. It does not support lookaheads, lookbehinds, or backreferences. Server-side RE2 validation is authoritative.

How It Works

1. You create a guardrail provider with provider_name: "regex".
2. You add one or more regex patterns.
3. You attach that provider to a guardrail rule.
4. The rule decides when to run the provider and whether to scan input, output, or both.
5. If a pattern matches, Bifrost applies that pattern’s action: detect only, block, or redact.

Custom Regex currently evaluates text content. It does not inspect image pixels or binary file contents.

Streaming output: When this profile is used in an output or both rule, Bifrost accumulates the stream until the model response is complete, then checks the full response. It does not check individual stream chunks. See Streaming Output Guardrails for details.

Pattern Fields

Field	Required	Description
pattern	Yes	RE2-compatible regex pattern
description	No	Human-readable label used in the intervention reason
entity_type	No	Redaction token to use for matches. Defaults to description when set, otherwise REGEX_MATCH.
flags	No	Optional flag or flag combination. Leave empty for no flag.
action	No	detect_only, block, or redact. Defaults to block.
redaction_strategy	No	replace, mask, or hash. Defaults to replace. Used when action is redact.
redaction_mode	No	runtime, logs_only, or runtime_reversible. Defaults to runtime. Used when action is redact.

The Web UI exposes these flag choices:

Flag	Meaning
None	No regex flag
i	Case-insensitive matching
m	Multiline mode
s	Dot matches newline
im	Case-insensitive + multiline
is	Case-insensitive + dot matches newline
ms	Multiline + dot matches newline
ims	Case-insensitive + multiline + dot matches newline

PII Detection Template

The Web UI includes a PII Detection template under Add Pattern. It pre-fills a Custom Regex configuration with common PII-like patterns:

Description	Flags
Email address	i
US phone number	-
US Social Security Number	-
Credit-card-like number	-
IPv4 address	-

The exact template patterns are shown in the config.json and Helm examples below.

The PII Detection template is pattern-based. It is fast and transparent, but it is not semantic PII classification. Expect some false positives and false negatives, especially for international phone numbers and unformatted values. For national IDs outside the US, add your own Custom Regex patterns for the exact country and format you need to enforce.

Redaction

Set a pattern’s action to redact when matching text should be rewritten instead of blocked. Custom Regex supports all Bifrost-managed redaction modes:

• runtime redacts the live request or response and stores the redacted value in logs.
• logs_only leaves runtime content raw but redacts Bifrost logs and trace-export connector content.
• runtime_reversible redacts runtime content and logs with reversible placeholders.

For the full behavior matrix, including reveal permissions and connector export behavior, see Guardrail Redaction.

Configuration

Web UI

1. Go to Guardrails > Providers.
2. Select Custom Regex.
3. Click Add Configuration.
4. Set a descriptive Name, for example pii-detection.
5. Click Add Pattern.
6. Choose either Custom regexp or PII Detection.
7. Enable the configuration and save it.
8. Attach the configuration to a guardrail rule under Guardrails > Configuration.

API

Create the Custom Regex provider configuration directly with the management API. The current Enterprise backend registers guardrail provider APIs at /api/guardrails/{provider}; the provider type is the path segment (regex), and the API assigns the configuration ID after creation.

curl -X POST http://localhost:8080/api/guardrails/regex \
  -H "Content-Type: application/json" \
  -d '{
    "name": "pii-detection",
    "enabled": true,
    "config": {
      "timeout": 5,
      "patterns": [
        {
          "pattern": "\\b[A-Z0-9._%+-]+@[A-Z0-9.-]+\\.[A-Z]{2,}\\b",
          "description": "Email address",
          "entity_type": "EMAIL",
          "flags": "i",
          "action": "redact",
          "redaction_strategy": "replace",
          "redaction_mode": "runtime_reversible"
        },
        {
          "pattern": "\\b(?:\\+?1[-.\\s]?)?(?:\\(?\\d{3}\\)?[-.\\s]?)\\d{3}[-.\\s]?\\d{4}\\b",
          "description": "US phone number",
          "entity_type": "PHONE_NUMBER",
          "action": "redact",
          "redaction_strategy": "replace",
          "redaction_mode": "runtime_reversible"
        },
        {
          "pattern": "\\b\\d{3}-\\d{2}-\\d{4}\\b",
          "description": "US Social Security Number",
          "entity_type": "US_SSN",
          "action": "redact",
          "redaction_strategy": "replace",
          "redaction_mode": "runtime_reversible"
        },
        {
          "pattern": "\\b(?:\\d[ -]?){13,19}\\b",
          "description": "Credit card-like number",
          "entity_type": "CREDIT_CARD",
          "action": "redact",
          "redaction_strategy": "replace",
          "redaction_mode": "runtime_reversible"
        },
        {
          "pattern": "\\b(?:25[0-5]|2[0-4]\\d|1\\d\\d|[1-9]?\\d)(?:\\.(?:25[0-5]|2[0-4]\\d|1\\d\\d|[1-9]?\\d)){3}\\b",
          "description": "IPv4 address",
          "entity_type": "IP_ADDRESS",
          "action": "redact",
          "redaction_strategy": "replace",
          "redaction_mode": "runtime_reversible"
        }
      ]
    }
  }'

To attach it through the API, fetch the generated ID with GET /api/guardrails/regex, then reference it in selectedGuardrailProfiles on POST /api/guardrails/rules.

config.json

{
  "guardrails_config": {
    "guardrail_providers": [
      {
        "id": 20,
        "provider_name": "regex",
        "policy_name": "pii-detection",
        "enabled": true,
        "timeout": 5,
        "config": {
          "patterns": [
            {
              "pattern": "\\b[A-Z0-9._%+-]+@[A-Z0-9.-]+\\.[A-Z]{2,}\\b",
              "description": "Email address",
              "entity_type": "EMAIL",
              "flags": "i",
              "action": "redact",
              "redaction_strategy": "replace",
              "redaction_mode": "runtime_reversible"
            },
            {
              "pattern": "\\b(?:\\+?1[-.\\s]?)?(?:\\(?\\d{3}\\)?[-.\\s]?)\\d{3}[-.\\s]?\\d{4}\\b",
              "description": "US phone number",
              "entity_type": "PHONE_NUMBER",
              "action": "redact",
              "redaction_strategy": "replace",
              "redaction_mode": "runtime_reversible"
            },
            {
              "pattern": "\\b\\d{3}-\\d{2}-\\d{4}\\b",
              "description": "US Social Security Number",
              "entity_type": "US_SSN",
              "action": "redact",
              "redaction_strategy": "replace",
              "redaction_mode": "runtime_reversible"
            },
            {
              "pattern": "\\b(?:\\d[ -]?){13,19}\\b",
              "description": "Credit card-like number",
              "entity_type": "CREDIT_CARD",
              "action": "redact",
              "redaction_strategy": "replace",
              "redaction_mode": "runtime_reversible"
            },
            {
              "pattern": "\\b(?:25[0-5]|2[0-4]\\d|1\\d\\d|[1-9]?\\d)(?:\\.(?:25[0-5]|2[0-4]\\d|1\\d\\d|[1-9]?\\d)){3}\\b",
              "description": "IPv4 address",
              "entity_type": "IP_ADDRESS",
              "action": "redact",
              "redaction_strategy": "replace",
              "redaction_mode": "runtime_reversible"
            }
          ]
        }
      }
    ],
    "guardrail_rules": [
      {
        "id": 201,
        "name": "block-pii-in-prompts",
        "description": "Block prompts containing common PII-like values",
        "enabled": true,
        "cel_expression": "headers[\"x-bf-tenant\"] == \"external\"",
        "query": {
          "combinator": "and",
          "rules": [
            {
              "field": "headers",
              "operator": "=",
              "value": "x-bf-tenant:external",
              "valueSource": "value"
            }
          ]
        },
        "apply_to": "input",
        "sampling_rate": 100,
        "timeout": 10,
        "provider_config_ids": [20]
      }
    ]
  }
}

Helm

bifrost:
  guardrails:
    providers:
      - id: 20
        provider_name: "regex"
        policy_name: "pii-detection"
        enabled: true
        timeout: 5
        config:
          patterns:
            - pattern: "\\b[A-Z0-9._%+-]+@[A-Z0-9.-]+\\.[A-Z]{2,}\\b"
              description: "Email address"
              entity_type: "EMAIL"
              flags: "i"
              action: "redact"
              redaction_strategy: "replace"
              redaction_mode: "runtime_reversible"
            - pattern: "\\b(?:\\+?1[-.\\s]?)?(?:\\(?\\d{3}\\)?[-.\\s]?)\\d{3}[-.\\s]?\\d{4}\\b"
              description: "US phone number"
              entity_type: "PHONE_NUMBER"
              action: "redact"
              redaction_strategy: "replace"
              redaction_mode: "runtime_reversible"
            - pattern: "\\b\\d{3}-\\d{2}-\\d{4}\\b"
              description: "US Social Security Number"
              entity_type: "US_SSN"
              action: "redact"
              redaction_strategy: "replace"
              redaction_mode: "runtime_reversible"
            - pattern: "\\b(?:\\d[ -]?){13,19}\\b"
              description: "Credit card-like number"
              entity_type: "CREDIT_CARD"
              action: "redact"
              redaction_strategy: "replace"
              redaction_mode: "runtime_reversible"
            - pattern: "\\b(?:25[0-5]|2[0-4]\\d|1\\d\\d|[1-9]?\\d)(?:\\.(?:25[0-5]|2[0-4]\\d|1\\d\\d|[1-9]?\\d)){3}\\b"
              description: "IPv4 address"
              entity_type: "IP_ADDRESS"
              action: "redact"
              redaction_strategy: "replace"
              redaction_mode: "runtime_reversible"

    rules:
      - id: 201
        name: "block-pii-in-prompts"
        description: "Block prompts containing common PII-like values"
        enabled: true
        cel_expression: 'headers["x-bf-tenant"] == "external"'
        query:
          combinator: "and"
          rules:
            - field: "headers"
              operator: "="
              value: "x-bf-tenant:external"
              valueSource: "value"
        apply_to: "input"
        sampling_rate: 100
        timeout: 10
        provider_config_ids: [20]

When To Use Custom Regex

Use Custom Regex when:

• You need transparent, deterministic matching.
• You want to block a known organization-specific identifier or data format.
• You want the built-in PII Detection template.
• You need a local guardrail with no external service dependency.

Use Secrets Detection instead when you want broad credential and API key coverage from the built-in Gitleaks rules.