RegexFilterProcessor

The RegexFilterProcessor applies zero-cost regex pattern matching to filter, redact, or block content in agent messages. No LLM calls are made — all detection is regex-based.

Supports built-in presets for common patterns (PII, secrets, URLs) and custom regex rules. Can be applied to input, output, or both phases.

Usage example

Block PII in input messages:

import { RegexFilterProcessor } from '@mastra/core/processors'

const filter = new RegexFilterProcessor({
  presets: ['pii'],
  strategy: 'block',
  phase: 'input',
})

Redact secrets in output:

import { RegexFilterProcessor } from '@mastra/core/processors'

const filter = new RegexFilterProcessor({
  presets: ['secrets'],
  strategy: 'redact',
  phase: 'output',
})

Custom rules:

import { RegexFilterProcessor } from '@mastra/core/processors'

const filter = new RegexFilterProcessor({
  rules: [{ name: 'internal-id', pattern: /INTERNAL-\d{6}/g, replacement: '[INTERNAL_ID]' }],
  strategy: 'redact',
})

Attach to an agent:

import { Agent } from '@mastra/core/agent'
import { RegexFilterProcessor } from '@mastra/core/processors'

const agent = new Agent({
  name: 'my-agent',
  model: 'openai/gpt-5-nano',
  processors: {
    input: [
      new RegexFilterProcessor({
        presets: ['pii', 'secrets'],
        strategy: 'block',
      }),
    ],
  },
})

Constructor parameters

rules?:

RegexRule[]

Custom regex rules to apply. Each rule has a name, a regex pattern, and an optional replacement string.

RegexRule

name:

string

Display name for the rule (used in match reports and error messages).

pattern:

RegExp

The regex pattern to match against.

replacement?:

string

Replacement string for redact strategy. Defaults to '[REDACTED]'.

presets?:

('pii' | 'secrets' | 'urls')[]

Built-in preset categories. 'pii' matches emails, phone numbers, SSNs, credit cards. 'secrets' matches API keys, bearer tokens, AWS keys. 'urls' matches HTTP/HTTPS URLs.

strategy?:

'block' | 'redact' | 'warn'

= 'block'

Strategy when a pattern match is found. 'block' aborts with a TripWire error. 'redact' replaces matched content with replacement text. 'warn' logs a warning but passes content through unchanged.

phase?:

'input' | 'output' | 'all'

= 'all'

Phases to apply the filter. 'input' filters input messages. 'output' filters output stream and result. 'all' filters both.

Returns

id:

'regex-filter'

Processor identifier.

name:

'Regex Filter'

Processor display name.

processInput:

(args: ProcessInputArgs) => ProcessInputResult

Checks input messages against all configured rules. Blocks, redacts, or warns depending on strategy. Skipped when phase is output.

processOutputStream:

(args: ProcessOutputStreamArgs) => Promise<ChunkType | null | undefined>

Checks streaming text-delta chunks against all configured rules. Skipped when phase is input.

processOutputResult:

(args: ProcessOutputResultArgs) => ProcessorMessageResult

Checks output messages against all configured rules. Blocks, redacts, or warns depending on strategy. Skipped when phase is input.

Error behavior

When the block strategy is active (default), RegexFilterProcessor throws a TripWire error with retry: false when any pattern matches. The TripWire metadata includes:

• processorId: 'regex-filter'
• matches: Array of match objects with rule, match (redacted to '[REDACTED_MATCH]'), and index
• strategy: 'block'

Built-in presets

Preset	Patterns	Default replacement
pii	Emails, phone numbers, SSNs, credit card numbers	[EMAIL], [PHONE], [SSN], [CREDIT_CARD]
secrets	API keys, bearer tokens, AWS access keys	[API_KEY], [BEARER_TOKEN], [AWS_KEY]
urls	HTTP/HTTPS URLs	[URL]