diff --git a/spec/params/params.spec.ts b/spec/params/params.spec.ts
index 19d18c83a..36dd96f5b 100644
--- a/spec/params/params.spec.ts
+++ b/spec/params/params.spec.ts
@@ -258,6 +258,34 @@ describe("Params value extraction", () => {
     expect(trueExpr.thenElse(twentytwo, 0).value()).to.equal(22);
     expect(falseExpr.thenElse(1, twentytwo).value()).to.equal(22);
   });
+
+  it("can select between RegExp/RegExp[] literals via a ternary expression", () => {
+    const localPattern = /^http:\/\/localhost:8080$/;
+    const prodPattern = /^https:\/\/example\.com$/;
+    const trueExpr = params.defineString("A_STRING").equals(params.defineString("SAME_STRING"));
+    const falseExpr = params.defineInt("AN_INT").equals(params.defineInt("DIFF_INT"));
+
+    expect(trueExpr.thenElse(localPattern, prodPattern).value()).to.equal(localPattern);
+    expect(falseExpr.thenElse(localPattern, prodPattern).value()).to.equal(prodPattern);
+
+    const localPatterns = [localPattern];
+    const prodPatterns = [prodPattern];
+    expect(trueExpr.thenElse(localPatterns, prodPatterns).value()).to.equal(localPatterns);
+    expect(falseExpr.thenElse(localPatterns, prodPatterns).value()).to.equal(prodPatterns);
+
+    // Nested thenElse, mirroring a boolean-param-selected CORS origin config.
+    // The outer test is false so that the nested expression is the one resolved.
+    const otherPatterns = [/^https:\/\/other\.example\.com$/];
+    const stagingExpr = params.defineBoolean("TRUE");
+    expect(
+      falseExpr.thenElse(localPatterns, stagingExpr.thenElse(prodPatterns, otherPatterns)).value()
+    ).to.equal(prodPatterns);
+    expect(
+      falseExpr
+        .thenElse(localPatterns, stagingExpr.equals(false).thenElse(prodPatterns, otherPatterns))
+        .value()
+    ).to.equal(otherPatterns);
+  });
 });
 
 describe("defineJsonSecret", () => {
@@ -457,6 +485,37 @@ describe("Params as CEL", () => {
       cmpExpr.thenElse(params.defineString("FOO"), params.defineString("BAR")).toCEL()
     ).to.equal("{{ params.A != params.B ? params.FOO : params.BAR }}");
   });
+
+  it("represents RegExp array branches as their string form, not '{}'", () => {
+    const booleanExpr = params.defineBoolean("BOOL");
+    const localPattern = /^http:\/\/localhost$/;
+    const prodPattern = /^https:\/\/example\.com$/;
+    const cel = booleanExpr.thenElse([localPattern], [prodPattern]).toCEL();
+
+    // Regression check: JSON.stringify(regexArray) alone would render each RegExp
+    // as "{}", silently dropping the pattern.
+    expect(cel).to.not.include("{}");
+    expect(cel).to.equal(
+      `{{ params.BOOL ? ${JSON.stringify([localPattern.toString()])} : ${JSON.stringify([
+        prodPattern.toString(),
+      ])} }}`
+    );
+  });
+
+  it("represents a bare RegExp branch as a quoted string form, not unquoted source", () => {
+    const booleanExpr = params.defineBoolean("BOOL");
+    const localPattern = /^http:\/\/localhost$/;
+    const prodPattern = /^https:\/\/example\.com$/;
+    const cel = booleanExpr.thenElse(localPattern, prodPattern).toCEL();
+
+    // Regression check: arg.toString() alone would render the RegExp as an
+    // unquoted /pattern/, which is not a valid CEL string literal.
+    expect(cel).to.equal(
+      `{{ params.BOOL ? ${JSON.stringify(localPattern.toString())} : ${JSON.stringify(
+        prodPattern.toString()
+      )} }}`
+    );
+  });
 });
 
 describe("expr template tag", () => {
diff --git a/spec/v2/providers/https.spec.ts b/spec/v2/providers/https.spec.ts
index 85930c36c..f4a91dcac 100644
--- a/spec/v2/providers/https.spec.ts
+++ b/spec/v2/providers/https.spec.ts
@@ -307,6 +307,89 @@ describe("onRequest", () => {
     }
   });
 
+  it("should allow a RegExp[] chosen dynamically via a ternary expression", async () => {
+    const isStaging = defineBoolean("IS_STAGING");
+    const localPattern = /^http:\/\/localhost:8080$/;
+    const stagingPattern = /^https:\/\/staging\.example\.com$/;
+
+    try {
+      process.env.IS_STAGING = "true";
+      const func = https.onRequest(
+        {
+          cors: isStaging.equals(true).thenElse([stagingPattern], [localPattern]),
+        },
+        (req, res) => {
+          res.send("42");
+        }
+      );
+      const req = request({
+        headers: {
+          referrer: "https://staging.example.com",
+          "content-type": "application/json",
+          origin: "https://staging.example.com",
+        },
+        method: "OPTIONS",
+      });
+
+      const response = await runHandler(func, req);
+
+      expect(response.status).to.equal(204);
+      expect(response.headers).to.deep.equal({
+        "Access-Control-Allow-Origin": "https://staging.example.com",
+        "Access-Control-Allow-Methods": "GET,HEAD,PUT,PATCH,POST,DELETE",
+        "Content-Length": "0",
+        Vary: "Origin, Access-Control-Request-Headers",
+      });
+    } finally {
+      delete process.env.IS_STAGING;
+      clearParams();
+    }
+  });
+
+  it("should resolve the other RegExp[] branch when the ternary expression is false", async () => {
+    const isStaging = defineBoolean("IS_STAGING");
+    const localPattern = /^http:\/\/localhost:8080$/;
+    const stagingPattern = /^https:\/\/staging\.example\.com$/;
+
+    try {
+      process.env.IS_STAGING = "false";
+      const func = https.onRequest(
+        {
+          cors: isStaging.equals(true).thenElse([stagingPattern], [localPattern]),
+        },
+        (req, res) => {
+          res.send("42");
+        }
+      );
+      const preflight = (origin: string) =>
+        request({
+          headers: {
+            referrer: origin,
+            "content-type": "application/json",
+            origin,
+          },
+          method: "OPTIONS",
+        });
+
+      const allowed = await runHandler(func, preflight("http://localhost:8080"));
+
+      expect(allowed.status).to.equal(204);
+      expect(allowed.headers).to.deep.equal({
+        "Access-Control-Allow-Origin": "http://localhost:8080",
+        "Access-Control-Allow-Methods": "GET,HEAD,PUT,PATCH,POST,DELETE",
+        "Content-Length": "0",
+        Vary: "Origin, Access-Control-Request-Headers",
+      });
+
+      const denied = await runHandler(func, preflight("https://staging.example.com"));
+
+      expect(denied.headers).to.not.have.property("Access-Control-Allow-Origin");
+    } finally {
+      delete process.env.IS_STAGING;
+      clearParams();
+    }
+  });
+
   it("should add CORS headers if debug feature is enabled", async () => {
     sinon.stub(debug, "isDebugFeatureEnabled").withArgs("enableCors").returns(true);
 
diff --git a/src/common/providers/https.ts b/src/common/providers/https.ts
index c636067a7..04f871630 100644
--- a/src/common/providers/https.ts
+++ b/src/common/providers/https.ts
@@ -715,6 +715,8 @@ export type CorsOption =
   | string
   | Expression<string>
   | Expression<string[]>
+  | Expression<RegExp>
+  | Expression<Array<string | RegExp>>
   | boolean
   | RegExp
   | Array<string | RegExp>;
diff --git a/src/params/index.ts b/src/params/index.ts
index e824f61fa..e130e9251 100644
--- a/src/params/index.ts
+++ b/src/params/index.ts
@@ -47,6 +47,7 @@ export type {
   SelectInput,
   SelectOptions,
   MultiSelectInput,
+  ExpressionValue,
   Param,
   SecretParam,
   JsonSecretParam,
diff --git a/src/params/types.ts b/src/params/types.ts
index c1e834b80..11e0a5678 100644
--- a/src/params/types.ts
+++ b/src/params/types.ts
@@ -24,12 +24,26 @@ import * as logger from "../logger";
 
 const EXPRESSION_TAG = Symbol.for("firebase-functions:Expression:Tag");
 
+/**
+ * The types an `Expression` can resolve to. `string`, `number`, `boolean` and
+ * `string[]` cover the values a param itself can hold; `RegExp` and
+ * `Array<string | RegExp>` are additionally allowed so that expressions can
+ * select between literals for options that accept them, such as `cors`.
+ */
+export type ExpressionValue =
+  | string
+  | number
+  | boolean
+  | string[]
+  | RegExp
+  | Array<string | RegExp>;
+
 /*
  * A CEL expression which can be evaluated during function deployment, and
  * resolved to a value of the generic type parameter: i.e, you can pass
  * an Expression<number> as the value of an option that normally accepts numbers.
  */
-export abstract class Expression<T extends string | number | boolean | string[]> {
+export abstract class Expression<T extends ExpressionValue> {
   /**
    * Handle the "Dual-Package Hazard" .
    *
@@ -144,13 +158,11 @@ export class TransformedStringExpression extends Expression<string> {
   }
 }
 
-export function valueOf<T extends string | number | boolean | string[]>(arg: T | Expression<T>): T {
+export function valueOf<T extends ExpressionValue>(arg: T | Expression<T>): T {
   return arg instanceof Expression ? arg.runtimeValue() : arg;
 }
 
-export function celOf<T extends string | number | boolean | string[]>(
-  arg: T | Expression<T>
-): T | string {
+export function celOf<T extends ExpressionValue>(arg: T | Expression<T>): T | string {
   return arg instanceof Expression ? arg.toCEL() : arg;
 }
 
@@ -171,13 +183,17 @@ export function transform(
  * - Arrays are represented as []-delimited, parsable JSON
  * - Numbers and booleans are not quoted explicitly
  */
-function refOf<T extends string | number | boolean | string[]>(arg: T | Expression<T>): string {
+function refOf<T extends ExpressionValue>(arg: T | Expression<T>): string {
   if (arg instanceof Expression) {
     return arg.toString();
   } else if (typeof arg === "string") {
     return `"${arg}"`;
+  } else if (arg instanceof RegExp) {
+    return JSON.stringify(arg.toString());
   } else if (Array.isArray(arg)) {
-    return JSON.stringify(arg);
+    // RegExp has no useful JSON representation (JSON.stringify(/foo/) === "{}"),
+    // so fall back to its string form instead of silently dropping the pattern.
+    return JSON.stringify(arg.map((item) => (item instanceof RegExp ? item.toString() : item)));
   } else {
     return arg.toString();
   }
@@ -186,9 +202,7 @@ function refOf<T extends string | number | boolean | string[]>(arg: T | Expressi
 /**
  * A CEL expression corresponding to a ternary operator, e.g {{ cond ? ifTrue : ifFalse }}
  */
-export class TernaryExpression<
-  T extends string | number | boolean | string[]
-> extends Expression<T> {
+export class TernaryExpression<T extends ExpressionValue> extends Expression<T> {
   constructor(
     private readonly test: Expression<boolean>,
     private readonly ifTrue: T | Expression<T>,
@@ -263,7 +277,7 @@ export class CompareExpression<
   }
 
   /** Returns a `TernaryExpression` which can resolve to one of two values, based on the resolution of this comparison. */
-  thenElse<retT extends string | number | boolean | string[]>(
+  thenElse<retT extends ExpressionValue>(
     ifTrue: retT | Expression<retT>,
     ifFalse: retT | Expression<retT>
   ) {
@@ -719,14 +733,11 @@ export class BooleanParam extends Param<boolean> {
   }
 
   /** @deprecated */
-  then<T extends string | number | boolean>(ifTrue: T | Expression<T>, ifFalse: T | Expression<T>) {
+  then<T extends ExpressionValue>(ifTrue: T | Expression<T>, ifFalse: T | Expression<T>) {
     return this.thenElse(ifTrue, ifFalse);
   }
 
-  thenElse<T extends string | number | boolean>(
-    ifTrue: T | Expression<T>,
-    ifFalse: T | Expression<T>
-  ) {
+  thenElse<T extends ExpressionValue>(ifTrue: T | Expression<T>, ifFalse: T | Expression<T>) {
     return new TernaryExpression(this, ifTrue, ifFalse);
   }
 }
diff --git a/src/v2/providers/https.ts b/src/v2/providers/https.ts
index 8aa9438ec..43e6dec53 100644
--- a/src/v2/providers/https.ts
+++ b/src/v2/providers/https.ts
@@ -76,13 +76,7 @@ export interface HttpsOptions extends Omit<GlobalOptions, "region" | "enforceApp
    * If this is an `Array`, allows requests from domains matching at least one entry of the array.
    * Defaults to true for {@link https.CallableFunction} and false otherwise.
    */
-  cors?:
-    | string
-    | Expression<string>
-    | Expression<string[]>
-    | boolean
-    | RegExp
-    | Array<string | RegExp>;
+  cors?: CorsOption;
 
   /**
    * Amount of memory to allocate to a function.