Prevent form submit if message contains a hyperlink using ajax, php and jquery

Question

Anyone able to help prevent form submittions that contain URL links in "message" field? Or better yet, let it go thru so (some) spammers think it went. Those submits which DO NOT contain a hyperlink get greenlighted to our inbox. Doable or pipedream?

HTML form in include php file:

<!-- Side form contact -->
 <form id="side-form" name="side-form" action="form-mailers.php" class="side-form" role="form" method="POST" onsubmit="return validateForm()">
  <h5 class="font-weight-bold mb-2 primary">Questions?</h5>
  <p class="serif mb-3">Ask the pros and receive the answers you seek.</p>
  <!-- Name -->
  <div class="form-row">
    <label for="name" class="sr-only"></label>
    <input type="text" name="name" class="form-control mb-3" id="name" placeholder="Name*">

  </div>
  <!-- Email -->
  <div class="form-row">
    <label for="email" class="sr-only"></label>
    <input type="email" name="email" class="form-control mb-3" id="email" placeholder="Email*">

  </div>
  <!-- Phone -->
  <div class="form-row">
    <label for="phone" class="sr-only"></label>
    <input type="tel" name="phone" class="form-control mb-3" id="phone" placeholder="Phone">
  </div>

  <!-- Subject -->
  <div class="form-group">
    <label for="subject" class="sr-only"></label>
    <select id="subject" class="custom-select browser-default" name="subject">
      <option value="">Choose topic</option>
      <option value="Feedback">Feedback</option>
      <option value="Issue">Report an Issue</option>
      <option value="NewOrder">New Order</option>
      <option value="Pricing">Price or Payment</option>
      <option value="Time">Timeline</option>
    </select>
    <div class="invalid-feedback">Please choose a topic</div>
  </div>
  <!-- Message -->
  <div class="form-group">
    <textarea class="form-control rounded-2" name="message" id="message" rows="3" placeholder="Message"></textarea>
  </div>
  <!-- Send button -->
  </form>
  <div class="center-on-small-only">
    <a class="btn btn-primary btn-md animate" onclick="validateForm()">Send Form</a>
  </div><div class="status" id="status"></div>

Validate JS @ bottom of webpage (not certain it's even used/needed):

// Validate Form AJAX (side-form)
function validateForm() {

    document.getElementById('status').innerHTML = "Sending...";
    formData = {
        'name'     : $('input[name=name]').val(),
        'email'    : $('input[name=email]').val(),
        'phone'    : $('input[name=phone]').val(),
        'subject'  : $('input[name=subject]').val(),
        'message'  : $('textarea[name=message]').val()
    };
   $.ajax({
    url : "form-mailers.php",
    type: "POST",
    data : formData,
    success: function(data, textStatus, jqXHR)
    {
        $('#status').text(data.message);
        if (data.code) //If mail was sent successfully, reset the form.
        $('#side-form').closest('form').find("input[type=text], input[type=email], textarea").val("");
    },
    error: function (jqXHR, textStatus, errorThrown)
    {
        $('#status').text(jqXHR);
    }
});
}

I have a form-mailers.php working fine:

<?php
$name = $_POST['name'];
$email = $_POST['email'];
$phone = $_POST['phone'];
$message = $_POST['message'];
$subject = $_POST['subject'];
header('Content-Type: application/json');
if ($name === ''){
print json_encode(array('message' => 'Name cannot be empty', 'code' => 0));
exit();
}
if ($email === ''){
print json_encode(array('message' => 'Email cannot be empty', 'code' => 0));
exit();
} else {
if (!filter_var($email, FILTER_VALIDATE_EMAIL)){
print json_encode(array('message' => 'Email format invalid.', 'code' => 0));
exit();
}
}
if ($subject === ''){
print json_encode(array('message' => 'Subject cannot be empty', 'code' => 0));
exit();
}
if ($message === ''){
print json_encode(array('message' => 'Message cannot be empty', 'code' => 0));
exit();
}
$content="From: $name \nEmail: $email \nPhone: $phone \nMessage: $message";
$recipient = "email_address_goes_here";
$mailheader = "From: $email \r\n";
mail($recipient, $subject, $content, $mailheader) or die("Error!");
print json_encode(array('message' => 'Email successfully sent!', 'code' => 1));
exit();
?>