diff --git a/server/test/unit/providers/network/advancedMatcher.test.ts b/server/test/unit/providers/network/advancedMatcher.test.ts
index f94e9bb53..2cfb3a7f0 100644
--- a/server/test/unit/providers/network/advancedMatcher.test.ts
+++ b/server/test/unit/providers/network/advancedMatcher.test.ts
@@ -119,6 +119,34 @@ describe("AdvancedMatcher", () => {
 				expect(result.ok).toBe(false);
 			});
 
+			it("returns a clear error when the regex pattern is rejected by RE2", () => {
+				const matcher = new AdvancedMatcher(createMockJmespath() as any);
+				const result = matcher.validate(
+					{ code: "200" },
+					// Backreferences are unsupported by RE2 and previously ReDoS-prone; RE2 throws on construction.
+					makeMonitor({ useAdvancedMatching: true, jsonPath: "code", matchMethod: "regex", expectedValue: "(a+)\\1" })
+				);
+
+				expect(result.ok).toBe(false);
+				expect(result.message).toBe("Invalid regex pattern");
+				expect(result.extracted).toBe("200");
+			});
+
+			it("returns a distinct error when the extracted value cannot be coerced to a string", () => {
+				// An object with a throwing toString/valueOf (like Object.create(null)) makes String() throw.
+				const unstringifiable = { toString: () => { throw new Error("nope"); } };
+				const jmespath = { search: jest.fn().mockReturnValue(unstringifiable) };
+				const matcher = new AdvancedMatcher(jmespath as any);
+
+				const result = matcher.validate(
+					{},
+					makeMonitor({ useAdvancedMatching: true, jsonPath: "field", matchMethod: "equal", expectedValue: "x" })
+				);
+
+				expect(result.ok).toBe(false);
+				expect(result.message).toBe("Error evaluating response value");
+			});
+
 			it("defaults to 'equal' comparison when matchMethod is undefined", () => {
 				const matcher = new AdvancedMatcher(createMockJmespath() as any);
 				const result = matcher.validate({ val: "exact" }, makeMonitor({ useAdvancedMatching: true, jsonPath: "val", expectedValue: "exact" }));
diff --git a/server/test/unit/validation/monitorValidation.test.ts b/server/test/unit/validation/monitorValidation.test.ts
index d7a68b00b..d6998367a 100644
--- a/server/test/unit/validation/monitorValidation.test.ts
+++ b/server/test/unit/validation/monitorValidation.test.ts
@@ -278,6 +278,53 @@ describe("monitorValidation — HEAD method gating", () => {
 	});
 });
 
+describe("monitorValidation — regex pattern gating", () => {
+	const baseRegex = {
+		name: "regex check",
+		type: "http" as const,
+		url: "https://example.com",
+		useAdvancedMatching: true,
+		matchMethod: "regex" as const,
+	};
+
+	describe("createMonitorBodyValidation", () => {
+		it("accepts an RE2-supported regex pattern", () => {
+			const parsed = createMonitorBodyValidation.parse({ ...baseRegex, expectedValue: "^2\\d{2}$" });
+			expect(parsed.expectedValue).toBe("^2\\d{2}$");
+		});
+
+		it("rejects a pattern using a backreference", () => {
+			expect(() => createMonitorBodyValidation.parse({ ...baseRegex, expectedValue: "(a+)\\1" })).toThrow();
+		});
+
+		it("rejects a pattern using lookahead", () => {
+			expect(() => createMonitorBodyValidation.parse({ ...baseRegex, expectedValue: "foo(?=bar)" })).toThrow();
+		});
+
+		it("ignores an invalid regex when matchMethod is not 'regex'", () => {
+			const parsed = createMonitorBodyValidation.parse({ ...baseRegex, matchMethod: "include", expectedValue: "(a+)\\1" });
+			expect(parsed.expectedValue).toBe("(a+)\\1");
+		});
+
+		it("ignores an empty expectedValue", () => {
+			const parsed = createMonitorBodyValidation.parse({ ...baseRegex, expectedValue: "" });
+			expect(parsed.matchMethod).toBe("regex");
+		});
+	});
+
+	describe("editMonitorBodyValidation", () => {
+		it("rejects a pattern using a backreference", () => {
+			expect(() => editMonitorBodyValidation.parse({ matchMethod: "regex", expectedValue: "(a+)\\1" })).toThrow();
+		});
+	});
+
+	describe("importMonitorsBodyValidation", () => {
+		it("rejects a pattern using a backreference on imported monitors", () => {
+			expect(() => importMonitorsBodyValidation.parse({ monitors: [{ ...baseRegex, expectedValue: "(a+)\\1" }] })).toThrow();
+		});
+	});
+});
+
 describe("monitorValidation — customUpCodes", () => {
 	const baseHttpBody = {
 		name: "HTTP check",