diff --git a/tests/lib/comment.js b/tests/lib/comment.js
new file mode 100644
index 0000000..a3c8236
--- /dev/null
+++ b/tests/lib/comment.js
@@ -0,0 +1,101 @@
+/**
+ * Copyright 2022 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import test from 'ava';
+import {
+  regexpParts,
+  transformEven,
+  isUrl,
+  rewriteCommentHrefs,
+} from '../../tools/lib/comment.js';
+
+test('regexpParts', (t) => {
+  let rp;
+
+  // Returns original text as single element in array if no match
+  rp = regexpParts('abc', /1/);
+  t.is(rp[0], 'abc');
+  t.is(rp.length, 1);
+
+  // Returns original text if no match
+  const match = `{@link ContentBounds}`;
+  const text = `Get the window's inner bounds as a ${match} object.`;
+  rp = regexpParts(text, /{@link.+?}/gs);
+  t.is(rp[1], match);
+  t.is(rp.length, 3);
+});
+
+test('transformEven', (t) => {
+  let te;
+  let arr = ['1', '2', '3', '4'];
+  const helper = (s) => s + '-';
+
+  // Modify even elements
+  te = transformEven(arr, helper);
+  for (let i = 0; i < te.length; i++) {
+    const addedByHelper = i % 2 === 0 ? '-' : '';
+    t.is(te[i], arr[i] + addedByHelper);
+  }
+});
+
+test('isUrl', async (t) => {
+  let iu;
+
+  // Valid URLs should return true
+  iu = isUrl('https://web.dev');
+  t.is(iu, true);
+  iu = isUrl('http://web.dev');
+  t.is(iu, true);
+  iu = isUrl('https://web.dev/learn');
+  t.is(iu, true);
+
+  // Incalid URLs should return false
+  iu = isUrl(':web.dev');
+  t.is(iu, false);
+  iu = isUrl('web.dev');
+  t.is(iu, false);
+});
+
+test('rewriteCommentHrefs', async (t) => {
+  let rch;
+
+  // rewrite href
+  const resolveHref = (path) => new URL(path, 'https://web.dev').toString();
+  const href = '/learn';
+  const contents = `Here's a link!`;
+  const rewrittenHref = resolveHref(href);
+  const text = `<a href="${href}">${contents}</a>`;
+  const rewrittenText = `<a href="${rewrittenHref}">${contents}</a>`;
+  rch = rewriteCommentHrefs(text, resolveHref);
+  t.is(rch, rewrittenText);
+
+  // throw when anchor has other properties
+  t.throws(() =>
+    rewriteCommentHrefs(`<a class="a" href="${href}"></a>`, resolveHref)
+  );
+  t.throws(() =>
+    rewriteCommentHrefs(`<a href="${href}" target="_blank"></a>`, resolveHref)
+  );
+
+  // If `resolveHref` function returns `undefined` then don't change link
+  // @ts-ignore
+  rch = rewriteCommentHrefs(text, () => undefined);
+  t.is(rch, text);
+
+  // If `resolveHref` function doesn't return a url create `@link`
+  rch = rewriteCommentHrefs(text, () => contents);
+  t.is(rch, `{@link ${contents}}`);
+});
diff --git a/tools/lib/comment.js b/tools/lib/comment.js
index 4297acc..a786bc7 100644
--- a/tools/lib/comment.js
+++ b/tools/lib/comment.js
@@ -38,7 +38,7 @@ const backtickSymbolRegexp = /`([\w\.]+)`/g;
  * @param {string} text
  * @param {RegExp} re
  */
-function regexpParts(text, re) {
+export function regexpParts(text, re) {
   /** @type {string[]} */
   const parts = [];
 
@@ -63,7 +63,7 @@ function regexpParts(text, re) {
  * @param {string[]} parts
  * @param {(s: string) => string} helper
  */
-function transformEven(parts, helper) {
+export function transformEven(parts, helper) {
   return parts.map((s, i) => {
     if ((i % 2) === 0) {
       return helper(s);
@@ -153,7 +153,7 @@ const hrefRegexp = /<a(\s+[^>]*?)href\s*=\s*(["'])(.*?)\2(.*?)>(.*?)<\/a>/gms;
  * @param {string} raw
  * @return {boolean} is this probably a URL?
  */
-function isUrl(raw) {
+export function isUrl(raw) {
   try {
     new URL(raw);
     return true;