$("#place_order").on('click',function(e){
    var mobile = $("#billing_phone").val();
    var pattern = /\+?(88)?01[3456789][0-9]{8}\b/;
    // return false;
	    if(!pattern.test(mobile) && mobile !== ''){
	        e.preventDefault();
			// $("#on_form").find(".phone-valid").after('<p class="on_warning">Please enter a valid mobile number.</p>');
			alert('Please enter a valid mobile number.');
	    }
  });