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

function isPresent(field) {
	return field.value.length > 0;
}

function emailIsValid(fieldID) {
	var field = document.getElementById(fieldID);
	if (!isPresent(field)) {
		alert('Please enter an email address.');
		field.focus();
		return false;
	}
	if (!isEmail(field)) {
		alert('That doesn\'t appear to be a valid email address; please try again.');
		field.focus();
		return false;
	}
	if (field.value == "infolist@londonalternativemarket.com") {
		alert("Thanks, but we're already on the mailing list ;-)");
		return false;
	}
	return true;
}