HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>jQuery scoped prefix selector</title>
  <style>
    form { border: 1px dashed #64748b; padding: 12px; margin: 10px 0; }
    input { display: block; margin: 6px 0; padding: 6px; width: 240px; }
    .billing-field { border: 2px solid #2563eb; background: #eff6ff; }
  </style>
</head>
<body>
  <form id="checkout">
    <input type="text" name="billing_email" placeholder="billing_email">
    <input type="text" name="billing_city" placeholder="billing_city">
    <input type="text" name="shipping_email" placeholder="shipping_email">
  </form>
  <form id="other">
    <input type="text" name="billing_email" placeholder="ignored (other form)">
  </form>
  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
  <script>
    $("#checkout input[name^='billing_']").addClass("billing-field");
  </script>
</body>
</html>

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jQuery #checkout input[name^='billing_'] (Try It)