How to validate Select field in amp-form to not allow default start option?

I have a select field that starts with an option that says "Select Location" and want to force the user to choose something before submitting. I saw this article about disabling the Submit button with amp-bind until an option is available, but I would like to use the validation built into amp-form if at all possible.

I have tried using the pattern attribute on the <select> and <option> fields. I have used something similar to pattern="^((?!default).)*$" and multiple variations without any success.

<form
    id="contactForm"
    method="post"
    action-xhr="https://valid.json/endpoint"
    custom-validation-reporting="show-all-on-submit"
    target="_top">

    [...]

    <select
        id="formLocation"
        name="location_id"
        pattern="^((?!default).)*$"
        required>
        <option value="default" disabled selected>Select Location</option>
        <option value="newyork">New York</option>
        <option value="losangeles">Los Angeles</option>
    </select>
    <span
        visible-when-invalid="patternMismatch"
        required
        validation-for="formLocation">
        Please Choose a Location
    </span>

    [...]

    <input
        id="formSubmit"
        type="submit"
        value="Submit">
</form>

When I click Submit without changing the value, I expect the validation error to appear, but it doesn't. Is it possible to use this validation method with Select fields? Or will I have to just use the aforementioned amp-bind method?

Solution

I am assuming that you have added all required script js files form the form. I have provided an example of the rating.

AMP is providing two types of validation one which is for the blank value and another when the pattern does not match. You are missing the blank value validation.

<script async custom-element="amp-form" src="https://cdn.ampproject.org/v0/amp-form-0.1.js"></script>


<form action-xhr="here will be your url" custom-validation-reporting="show-all-on-submit" target="_top" method="post" class="wpcf7-form i-amphtml-form amp-form-dirty" novalidate="">

    <label for="rating">Select rating</label>
    <select name="rating" required="" class="user-invalid valueMissing" id="show-all-on-submit-select" aria-invalid="false">
        <option value="">Rate…</option>
        <option value="5">Perfect</option>
        <option value="4">Good</option>
        <option value="3">Average</option>
        <option value="2">Not that bad</option>
        <option value="1">Very poor</option>
    </select>
    // You are missing this one 
    <span visible-when-invalid="valueMissing" validation-for="show-all-on-submit-select">

         Please select rating..!!
    </span>
    // This is for the pattern validation message. If the field is having the value but not does not match with patter this span will provide the validation
    <span visible-when-invalid="patternMismatch" validation-for="show-all-on-submit-select">
            Please select rating..!!
    </span>

    <input type="submit" name="submit" value="Submit" class="wpcf7-form-control wpcf7-submit button yellow-button">

</form>

Now, If you need the same solution in your code just put below span I think it must work for you:

 <span
    visible-when-invalid="valueMissing"
    required
    validation-for="formLocation" 
    validation-for="show-all-on-submit-select">
    Please Choose a Location
 </span>

Thanks