在确认模式和提交表单之前验证表单字段

我试图在表单提交之前使用Bootstrap Modal作为我的确认对话框。然而，我想验证首先我的形式显示模态之前。我该怎么做呢?

这是我到目前为止所尝试的:

<script type="text/javascript">
    $(function(){
        $(".btn-show-modal").click(function(e){ 
            e.preventDefault(); 
            var id = $(this).attr('id'); 
            var modal_id = "dialog-example_"+id; 
            $("#"+modal_id).modal('show'); 
        });
        $(".btn btn-custom").click(function(e) {
            var id = $(this).attr('id');
            var modal_id = "dialog-example_"+id;
            $("#reservationForm").submit();
            $("#"+modal_id).modal('hide');
        });
    });
</script>
<form class="form" method="POST" action="unit-reservation.php" id="reservationForm">
    <div class="form-horizontal">
        <div class="control-group">
            <label class="control-label"><b>Firstname (*):</b></label>
            <div class="controls">
                <input type="text" name="customerfname" required />
            </div>
        </div>
        <div class="control-group">
            <label class="control-label"><b>Middlename (*):</b></label>
            <div class="controls">
                <input type="text" name="customermname" required />
            </div>
        </div>
        <div class="control-group">
            <label class="control-label"><b>Lastname (*):</b></label>
            <div class="controls">
                <input type="text" name="customerlname" required />
            </div>
        </div>
        <div class="control-group">
            <label class="control-label"><b>Email Address (*):</b></label>
            <div class="controls">
                <input type="email" name="customeremailaddress" required />
            </div>
        </div>
        <div class="control-group">
            <label class="control-label"><b>Address:</b></label>
            <div class="controls">
                <input type="text" name="customeraddress"  required />
            </div>
        </div>
        <div class="control-group">
            <label class="control-label"><b>Telephone Number:</b></label>
            <div class="controls">
                <input type="tel" name="customertelnumber"  />
            </div>
        </div>
        <div class="control-group">
            <label class="control-label"><b>Mobile Number:</b></label>
            <div class="controls">
                <input type="number" step="any" min="0" name="customermobilenumber" required /
            </div>
        </div>
    </div>
    <input type="hidden" value="<?php echo $user; ?>" name="user_id">
    <button id="<?php echo $code; ?>" class="btn btn-custom btn-show-modal" data-toggle="modal">Reserve Now</button>          
</form>
<div class="modal hide fade" id="dialog-example_<?php echo $code; ?>">
    <div class="modal-header">
        <h5>Confirm Reservation</h5>
    </div>
    <div class="modal-body">
        <p class="modaltext">Are you sure you want to reserve unit: <?php echo $code; ?>?</p>
    </div>
    <div class="modal-footer">
        <a href="#" data-dismiss="modal" class="btn">No</a>
        <a href="" class="btn btn-custom" id="<?php echo $code ;?>">Yes</a>
    </div>
</div>