Form Validation – Input Value Must Contain Number Higher Than 5

Please see the original Magento contact form controller, i.e. app/code/core/Mage/Contacts/controllers/IndexController.php -> public function postAction(). For example you have to add this in your sendemailAction function below $params = $this->getRequest()->getParams(); (don't forget to surround it with try/catch block):

        $error = false;

        if (!Zend_Validate::is(trim($params['name']) , 'NotEmpty')) {
            $error = true;
        }

        if (!Zend_Validate::is(trim($params['email']), 'EmailAddress')) {
            $error = true;
        }

        if ($error) {
            throw new Exception();
        }

This is PHP validation. If you like JavaScript validation - you can see again Magento original contact form for reference, i.e. app/design/frontend/base/default/template/contacts/form.phtml. So you have to add this at the bottom of your form:

<script type="text/javascript">
//<![CDATA[
    var simpleContactForm = new VarienForm('simple_contact_form', true);
//]]>
</script>

You can use pattern for validate by your own regular expression:

data-validate="{'required':true, 'pattern':/^\d+(\.\d{1,2})?$/, 'validate-greater-than-zero':true}"

You can check validate form using .valid() method:

<script>
    require(['jquery'], function($) {
        $(function() {
            $('#form-id').submit(function() {
                if($('#form-id').valid()) {
                    ... ajax query ...
                }
                return false;
            });
        });
    });
</script>