diff --git a/README.md b/README.md
index a6a9d86..1b71569 100644
--- a/README.md
+++ b/README.md
@@ -204,21 +204,17 @@ unnamed wildcards are not collected.
 ## make pattern from regex
 
 ```javascript
-> const pattern = new UrlPattern(/^\/api\/(.*)$/);
-```
-
-if the pattern was created from a regex an array of the captured groups is returned on a match:
+> const pattern = new UrlPattern(/^\/api\/(.*)$/, ["path"]);
 
-```javascript
 > pattern.match("/api/users");
-["users"]
+{path: "users"}
 
 > pattern.match("/apiii/test");
 undefined
 ```
 
 when making a pattern from a regex
-you can pass an array of keys as the second argument.
+you have to pass an array of keys as the second argument.
 returns objects on match with each key mapped to a captured value:
 
 ```javascript
diff --git a/src/url-pattern.ts b/src/url-pattern.ts
index 04dca2b..a457b31 100644
--- a/src/url-pattern.ts
+++ b/src/url-pattern.ts
@@ -26,7 +26,7 @@ export default class UrlPattern {
   public readonly isRegex: boolean;
   public readonly regex: RegExp;
   public readonly ast?: Array<Ast<any>>;
-  public readonly names?: string[];
+  public readonly names: string[];
 
   constructor(pattern: string, options?: IUserInputOptions);
   constructor(pattern: RegExp, groupNames?: string[]);
@@ -51,22 +51,25 @@ export default class UrlPattern {
     // handle regex pattern and return early
     if (pattern instanceof RegExp) {
       this.regex = pattern;
-      if (optionsOrGroupNames != null) {
-        if (!Array.isArray(optionsOrGroupNames)) {
-          throw new TypeError([
-            "if first argument is a RegExp the second argument",
-            "may be an Array<String> of group names",
-            "but you provided something else",
-          ].join(" "));
-        }
-        const groupCount = regexGroupCount(this.regex);
-        if (optionsOrGroupNames.length !== groupCount) {
-          throw new Error([
-            `regex contains ${ groupCount } groups`,
-            `but array of group names contains ${ optionsOrGroupNames.length }`,
-          ].join(" "));
-        }
-        this.names = optionsOrGroupNames;
+      if (optionsOrGroupNames == null || !Array.isArray(optionsOrGroupNames)) {
+        throw new TypeError([
+          "if first argument is a RegExp the second argument",
+          "must be an Array<String> of group names",
+        ].join(" "));
+      }
+      const groupCount = regexGroupCount(this.regex);
+      if (optionsOrGroupNames.length !== groupCount) {
+        throw new Error([
+          `regex contains ${ groupCount } groups`,
+          `but array of group names contains ${ optionsOrGroupNames.length }`,
+        ].join(" "));
+      }
+      this.names = optionsOrGroupNames;
+      const regexNameIndex = indexOfDuplicateElement(this.names);
+      if (regexNameIndex !== -1) {
+        throw new Error(
+          `duplicate name "${ this.names[regexNameIndex] }" in pattern. names must be unique`,
+        );
       }
       return;
     }
@@ -114,24 +117,20 @@ export default class UrlPattern {
     }
   }
 
-  public match(url: string): { [index: string]: string } | string[] | undefined {
+  public match(url: string): { [index: string]: string } | undefined {
     const match = this.regex.exec(url);
     if (match == null) {
       return;
     }
 
     const groups = match.slice(1);
-    if (this.names != null) {
-      const mergedNamesAndGroups: { [index: string]: string } = {};
-      for (let i = 0; i < this.names.length; i++) {
-        if (groups[i] != null) {
-          mergedNamesAndGroups[this.names[i]] = groups[i];
-        }
+    const mergedNamesAndGroups: { [index: string]: string } = {};
+    for (let i = 0; i < this.names.length; i++) {
+      if (groups[i] != null) {
+        mergedNamesAndGroups[this.names[i]] = groups[i];
       }
-      return mergedNamesAndGroups;
-    } else {
-      return groups;
     }
+    return mergedNamesAndGroups;
   }
 
   public stringify(params?: object): string {
diff --git a/test/errors.ts b/test/errors.ts
index eb1b496..c28e990 100644
--- a/test/errors.ts
+++ b/test/errors.ts
@@ -117,10 +117,9 @@ tape("regex names", (t: tape.Test) => {
   try {
     new UntypedUrlPattern(/x/, 5);
   } catch (error) {
-    t.equal(error.message, [
-      "if first argument is a RegExp the second argument may be an Array<String>",
-      "of group names but you provided something else",
-    ].join(" "));
+    t.equal(error.message,
+      "if first argument is a RegExp the second argument must be an Array<String> of group names",
+    );
   }
   try {
     new UrlPattern(/(((foo)bar(boo))far)/, []);
@@ -137,7 +136,7 @@ tape("regex names", (t: tape.Test) => {
 
 tape("stringify regex", (t: tape.Test) => {
   t.plan(1);
-  const pattern = new UrlPattern(/x/);
+  const pattern = new UrlPattern(/x/, []);
   try {
     pattern.stringify();
   } catch (error) {
diff --git a/test/match-fixtures.ts b/test/match-fixtures.ts
index c69ac38..58f6ad8 100644
--- a/test/match-fixtures.ts
+++ b/test/match-fixtures.ts
@@ -24,14 +24,12 @@ tape("match", (t: tape.Test) => {
   pattern = new UrlPattern(".foo");
   t.equals(pattern.match(".bar.foo"), undefined);
 
-  pattern = new UrlPattern(/foo/);
+  pattern = new UrlPattern(/foo/, []);
   t.deepEqual(pattern.match("foo"), []);
 
-  pattern = new UrlPattern(/\/foo\/(.*)/);
-  t.deepEqual(pattern.match("/foo/bar"), ["bar"]);
-
-  pattern = new UrlPattern(/\/foo\/(.*)/);
-  t.deepEqual(pattern.match("/foo/"), [""]);
+  pattern = new UrlPattern(/\/foo\/(.*)/, ["path"]);
+  t.deepEqual(pattern.match("/foo/bar"), {path: "bar"});
+  t.deepEqual(pattern.match("/foo/"), {path: ""});
 
   pattern = new UrlPattern("/user/:userId/task/:taskId");
   t.deepEqual(pattern.match("/user/10/task/52"), {
@@ -308,27 +306,7 @@ tape("match", (t: tape.Test) => {
     scheme: "https",
   });
 
-  let regex = /\/ip\/(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$/;
-  pattern = new UrlPattern(regex);
-  t.equal(undefined, pattern.match("10.10.10.10"));
-  t.equal(undefined, pattern.match("ip/10.10.10.10"));
-  t.equal(undefined, pattern.match("/ip/10.10.10."));
-  t.equal(undefined, pattern.match("/ip/10."));
-  t.equal(undefined, pattern.match("/ip/"));
-  t.deepEqual(pattern.match("/ip/10.10.10.10"), ["10", "10", "10", "10"]);
-  t.deepEqual(pattern.match("/ip/127.0.0.1"), ["127", "0", "0", "1"]);
-
-  regex = /\/ip\/((?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?))$/;
-  pattern = new UrlPattern(regex);
-  t.equal(undefined, pattern.match("10.10.10.10"));
-  t.equal(undefined, pattern.match("ip/10.10.10.10"));
-  t.equal(undefined, pattern.match("/ip/10.10.10."));
-  t.equal(undefined, pattern.match("/ip/10."));
-  t.equal(undefined, pattern.match("/ip/"));
-  t.deepEqual(pattern.match("/ip/10.10.10.10"), ["10.10.10.10"]);
-  t.deepEqual(pattern.match("/ip/127.0.0.1"), ["127.0.0.1"]);
-
-  regex = /\/ip\/((?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?))$/;
+  const regex = /\/ip\/((?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?))$/;
   pattern = new UrlPattern(regex, ["ip"]);
   t.equal(undefined, pattern.match("10.10.10.10"));
   t.equal(undefined, pattern.match("ip/10.10.10.10"));
diff --git a/test/readme.ts b/test/readme.ts
index 39b9655..f28891e 100644
--- a/test/readme.ts
+++ b/test/readme.ts
@@ -74,8 +74,8 @@ tape("domain example", (t: tape.Test) => {
 });
 
 tape("regex example", (t: tape.Test) => {
-  const pattern = new UrlPattern(/^\/api\/(.*)$/);
-  t.deepEqual(pattern.match("/api/users"), ["users"]);
+  const pattern = new UrlPattern(/^\/api\/(.*)$/, ["path"]);
+  t.deepEqual(pattern.match("/api/users"), {path: "users"});
   t.equal(pattern.match("/apiii/users"), undefined);
   t.end();
 });