> ## Documentation Index > Fetch the complete documentation index at: https://docs.mcpmanager.ai/llms.txt > Use this file to discover all available pages before exploring further. # Regex > How regular-expression gateway rules work in MCP Manager: JavaScript regex syntax with case-insensitive global matching, multiple OR patterns, the five actions (block, redact, replace, mask, hash), and ready-to-use patterns for prompt injection, SSNs, credit cards, and secrets. A **regular expression** rule is the most flexible [gateway rule](/features/gateway-rules/overview) 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. ## 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](/features/gateway-rules/overview#detection-hook-when-a-rule-fires) 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](/features/gateway-rules/overview#failure-mode-what-happens-when-a-detection-method-fails). ## 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. ## 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](https://regex101.com) and paste the corrected version back. ## Actions Regular-expression rules support **all five** rule actions. The action applies to the text each pattern matched: | Action | What it does to the matched text | | ----------- | --------------------------------------------------------------------------------------------------------------------------------------------- | | **Block** | Blocks the whole message. A blocked request never reaches the server; a blocked response never reaches the client. | | **Redact** | Removes the match entirely, leaving nothing in its place. | | **Replace** | Substitutes the match with the constant ``. | | **Mask** | Replaces each character of the match with an asterisk, preserving the original length. | | **Hash** | Replaces the match with a truncated SHA-256 hash, `` (16 hex characters), so you can correlate repeated values without exposing them. | 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](/features/gateway-rules/overview#actions-what-a-matching-rule-does) for how actions and [rule order](/features/gateway-rules/overview#rule-order-and-the-enable-toggle) interact. ## Examples Detection method: **Regular expression** · Action: **Block** · Alerts: **on** ```text Patterns theme={null} ignore\s+(all\s+)?(previous|prior|above|earlier)\s+(instructions|prompts|directives) you\s+are\s+now\s+(in\s+)?(developer|admin|debug|unrestricted)\s+mode disregard\s+(all\s+)?(your|the)\s+(previous|prior|safety|system)\s+(instructions|rules|guidelines|prompt) system\s*:\s*(you\s+are|from\s+now|new\s+instructions|override) ``` 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** ```text Pattern theme={null} \b\d{3}[-\s]?\d{2}[-\s]?\d{4}\b ``` Matches `123-45-6789`, `123 45 6789`, and `123456789`. With **Replace** each match becomes ``; with **Redact** it disappears. Detection method: **Regular expression** · Action: **Mask** ```text Pattern theme={null} \b(?:\d[ -]*?){13,19}\b ``` Catches most card formats and replaces the digits with asterisks, preserving length. For checksum-validated detection with fewer false positives, use a [Presidio](/features/gateway-rules/presidio) rule with the `CREDIT_CARD` entity instead. Detection method: **Regular expression** · Action: **Replace** ```text Patterns theme={null} (?:api[_-]?key|api[_-]?secret|access[_-]?token|auth[_-]?token)\s*[:=]\s*['"]?[A-Za-z0-9_\-\.]{20,}['"]? sk[-_]live[-_][A-Za-z0-9]{20,} ghp_[A-Za-z0-9]{36,} AKIA[0-9A-Z]{16} ``` 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](/features/alerts) and [logs](/features/viewing-logs) to see what it catches, tune the pattern to remove false positives, and only then switch high-severity rules to **Block**. ## Further reading Context-aware detection for unstructured PII like names and addresses. Detection methods, hooks, failure modes, actions, and rule ordering. Delegate nuanced policy to AWS Bedrock, Lakera Guard, or your own webhook.