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A regular expression rule is the most flexible gateway rule detection method in MCP Manager: you supply one or more patterns, and the rule matches them against the text of a tool message. Select Regular expression as the Detection method in the rule editor on a gateway’s Rules tab.

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How regex matching works

MCP Manager compiles each pattern as a JavaScript regular expression and evaluates it with the case-insensitive (i) and global (g) flags. Matching therefore ignores letter case and finds every occurrence in the message, not just the first. Enter patterns in JavaScript syntax (the same syntax the RegExp constructor accepts); surrounding slashes are optional. A regex rule scans the text of the tool message on whichever detection hook you chose — the tool’s arguments on the request leg, or the tool’s result on the response leg. Because regex runs in-process and synchronously, it never “fails,” so a regex rule has no failure mode.

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Multiple patterns

A single rule can hold more than one pattern. Use Add matching pattern in the rule editor to add another. Patterns are evaluated as an OR: if any pattern matches, the rule’s action fires. Each pattern is compiled and tested independently.

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Pattern validation and the Regex101 helper

If a pattern has invalid syntax, the rule editor shows an inline error with a “Click here to test and fix your pattern on Regex101” link, pre-filled with your pattern so you can debug it on regex101.com and paste the corrected version back.

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Actions

Regular-expression rules support all five rule actions. The action applies to the text each pattern matched: For the modification actions (redact, replace, mask, hash), every occurrence of every matched pattern is transformed and the message then continues to the next enabled rule. A Block action stops rule processing immediately. See Actions for how actions and rule order interact.

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Examples

Detection method: Regular expression · Action: Block · Alerts: on
Patterns
If a tool response carries text like “ignore your previous instructions,” the response is blocked before the model sees it. Enable alerts so you’re notified on every attempt.
Detection method: Regular expression · Action: Replace
Pattern
Matches 123-45-6789, 123 45 6789, and 123456789. With Replace each match becomes <SENSITIVE>; with Redact it disappears.
Detection method: Regular expression · Action: Mask
Pattern
Catches most card formats and replaces the digits with asterisks, preserving length. For checksum-validated detection with fewer false positives, use a Presidio rule with the CREDIT_CARD entity instead.
Detection method: Regular expression · Action: Replace
Patterns
Targets generic key/secret assignments plus Stripe secret keys, GitHub personal access tokens, and AWS access key IDs.
Roll a new pattern out on a non-destructive action first. Set the action to Replace with Alerts on, watch the Alerts and logs to see what it catches, tune the pattern to remove false positives, and only then switch high-severity rules to Block.

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Further reading

Microsoft Presidio

Context-aware detection for unstructured PII like names and addresses.

Gateway Rules Overview

Detection methods, hooks, failure modes, actions, and rule ordering.

Custom Rule Engines

Delegate nuanced policy to AWS Bedrock, Lakera Guard, or your own webhook.