public String validate() {
	String error = null;
	
	if(StringUtils.isEmpty(username)) {
		error = "You have to enter a username";
	} else if(!EMAIL_ADDRESS_REGEX.matcher(email).matches()) {
		error = "You have to enter a valid email address";
	} else if(StringUtils.isEmpty(password)) {
		error = "You have to enter a password";
	} else if(!password.equals(password2)) {
		error = "The two passwords do not match";
	}
	
	return error;
}