function check_email(e) {
	var filter = /^([a-zA-Z0-9_\.\-])+\@(([a-zA-Z0-9\-])+\.)+([a-zA-Z0-9]{2,4})+$/;
	return filter.test(e);
}

function mark_field(form, message){
	window.alert(message);
	form.focus();
	form.style.borderColor = '#ffaaaa';
	form.style.background = '#ffdddd';
}

function clear_marking(form){
	form.style.borderColor = '';
	form.style.background = '';
}

function send_form (sender) {

	var form = document.contact_form;

	if (form.sender_name.value == '') {
		mark_field(form.sender_name, 'Nie podano imienia i nazwiska');
		return false;
	}

	if (form.phone_number.value == '' ) {
		mark_field(form.phone_number, 'Nie podano numeru telefonu kontaktowego');
		return false;
	}

	if (!check_email(form.email.value) ) {
		mark_field(form.email, 'Podany adres email jest niepoprawny');
		return false;
	}

	if (form.message.value == '' ) {
		mark_field(form.message, 'Nie podano treści wiadomości');
		return false;
	}

	sender.submit();
}