regex101: Prefix all classes

Regular Expression

(?<=class=["'][\w\- :\.]*)([\w\-:\.]+)

New

<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">

Substitution

<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">