const regex = /(?<=class=["'][\w\- :\.]*)([\w\-:\.]+)/gm;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('(?<=class=["\'][\\w\\- :\\.]*)([\\w\\-:\\.]+)', 'gm')
const str = `<div class="px-4 sm:px-6 lg:px-8">
<div class="sm:flex sm:items-center">
<div class="sm:flex-auto">
<h1 class="text-xl font-semibold text-gray-900">Users</h1>
<p class="mt-2 text-sm text-gray-700">A list of all the users in your account including their name, title, email and role.</p>
</div>
<div class="mt-4 sm:mt-0 sm:ml-16 sm:flex-none">
<button type="button" class="inline-flex items-center justify-center rounded-md border border-transparent bg-indigo-600 px-4 py-2 text-sm font-medium text-white shadow-sm hover:bg-indigo-700 focus:outline-none focus:ring-2 focus:ring-indigo-500 focus:ring-offset-2 sm:w-auto">Add user</button>
</div>
</div>
<div class="mt-8 flex flex-col">
<div class="-my-2 -mx-4 overflow-x-auto sm:-mx-6 lg:-mx-8">
<div class="inline-block min-w-full py-2 align-middle">
<div class="overflow-hidden shadow-sm ring-1 ring-black ring-opacity-5">
<table class="min-w-full divide-y divide-gray-300">
<thead class="bg-gray-50">
<tr>
<th scope="col" class="py-3.5 pl-4 pr-3 text-left text-sm font-semibold text-gray-900 sm:pl-6 lg:pl-8">Name</th>
<th scope="col" class="px-3 py-3.5 text-left text-sm font-semibold text-gray-900">Title</th>
<th scope="col" class="px-3 py-3.5 text-left text-sm font-semibold text-gray-900">Email</th>
<th scope="col" class="px-3 py-3.5 text-left text-sm font-semibold text-gray-900">Role</th>
<th scope="col" class="relative py-3.5 pl-3 pr-4 sm:pr-6 lg:pr-8">
<span class="sr-only">Edit</span>
</th>
</tr>
</thead>
<tbody class="divide-y divide-gray-200 bg-white">
<tr>
<td class="whitespace-nowrap py-4 pl-4 pr-3 text-sm font-medium text-gray-900 sm:pl-6 lg:pl-8">Lindsay Walton</td>
<td class="whitespace-nowrap px-3 py-4 text-sm text-gray-500">Front-end Developer</td>
<td class="whitespace-nowrap px-3 py-4 text-sm text-gray-500">lindsay.walton@example.com</td>
<td class="whitespace-nowrap px-3 py-4 text-sm text-gray-500">Member</td>
<td class="relative whitespace-nowrap py-4 pl-3 pr-4 text-right text-sm font-medium sm:pr-6 lg:pr-8">
<a href="#" class="text-indigo-600 hover:text-indigo-900">Edit<span class="sr-only">, Lindsay Walton</span></a>
</td>
</tr>
<!-- More people... -->
</tbody>
</table>
</div>
</div>
</div>
</div>
</div>`;
const subst = ``;
// The substituted value will be contained in the result variable
const result = str.replace(regex, subst);
console.log('Substitution result: ', result);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for JavaScript, please visit: https://developer.mozilla.org/en/docs/Web/JavaScript/Guide/Regular_Expressions