rel is just a random attribute... it could be anything. It sounds like you would want both the country name and ID in the option so that you could populate a hidden form element with the ID and the innerhtml of a span with the name.
So the code might look like:
<script>
function getCountryData(list) {
var listOption = list.options[list.selectedIndex];
var countryID = listOption.getAttribute("data-country-id");
var countryName = listOption.getAttribute("data-country");
document.getElementById("countryID").value = countryID;
document.getElementById("showCountry").innerHTML= countryID;
}
</script>
<select id="regionID" name="regionID" class="form-control" onchange="getCountryData(this)">
<?php
while(!$rsREGIONS->atEnd()) {
?>
<option value="<?php echo($rsREGIONS->getColumnVal("regionID")); ?>" data-country="<?php echo($rsREGIONS->getColumnVal('country')); ?>" data-country-id="<?php echo($rsREGIONS->getColumnVal('country')); ?>"><?php echo($rsREGIONS->getColumnVal("region")); ?></option>
<?php
$rsREGIONS->moveNext();
}
$rsREGIONS->moveFirst(); //return RS to first record
?>
</select>
<input type="hidden" id="countryID" name="countryID" value="">
<span id="showCountry"></span>