function clearText(field) {
    if (field.defaultValue == field.value) field.value = '';
    else if (field.value == '') field.value = field.defaultValue;
}

function isEmpty(textField) {

	with(textField) {
		if(value == null || value == "") {
			return true;
		}
	}

    return false;
}

function isValidEmail(emailField) {

	var reg = /^([A-Za-z0-9_\-\.])+\@([A-Za-z0-9_\-\.])+\.([A-Za-z]{2,4})$/;

	with(emailField) {
		if(reg.test(value) == false) {
			return false;
		}
	}

	return true;
}

function validateForm(form) {

	var message = "The comment cannot be posted, because:\n\n";
	var invalid = false;

	if(isEmpty(form.author)) {
		message += "- You have not entered your name.\n";
		invalid = true;
	}

	if(!isValidEmail(form.email)) {
		message += "- You have entered an invalid email.\n";
		invalid = true;
	}

	if(isEmpty(form.comment)) {
		message += "- You cannot post an empty comment.\n";
		invalid = true;
	}

	if(invalid) {
		alert(message);
		return false;
	}

	return true;
}