How can i use javascript to split a domain name into two variables on a form before posting?

I have a an input box on my homepage which i want users to input their domain to search for . E.G.

test.com

or

test.gb.com

etc and i would like it to on submission ( post ) to extract the everything upto the first "." into a variable $domain and everything after the first including "." into $ext

So $domain might now contain "test" and $ext would contain ".gb.com" When the form was posted.

Possible ?

Here is the code for my form so far . What i need is to have everything that is inputted upto the first "." stored in the name="domain" form variable and ever开发者_如何学运维ything in this line i.e from the $ext variable stored for the form submission

Code so far :

<form method="post" action="https://www.jms-networks.net/domainchecker.php">
<span class="text"><input value="{php} print $_POST['sld'] ? $_POST['sld'] : 'Search for your domain name'; {/php}" type="text" name="domain" class="searchbox" onfocus="if (this.value == this.defaultValue) this.value = '';" onblur="if (this.value == '') this.value = this.defaultValue;" /></span>
<input type="hidden" name="direct" value="true" />
<!-- This one below should contain the $ext value everythign after including the first "." -->
<input type="hidden" name="tlds[]" value="what goes here???" />
<input type="hidden" name="tlds[]" value=".me.uk" />
<input type="hidden" name="tlds[]" value=".org.uk" />
<input type="hidden" name="tlds[]" value=".com" />
<input type="hidden" name="tlds[]" value=".net" />
<input type="hidden" name="tlds[]" value=".org" />

<input type="submit" value="Go" class="product-headers" />
</form>

var i = $thewhole.indexOf('.');
var $domain = $thewhole.substr(0, i);
var $ext = $thewhole.substr(i);

Or with regexp

var regexp = /([^.]+)(\..*)/;
var match = regexp.exec($thewhole);
var $domain = match[1];
var $ext = match[2];

Edit:

<script type="text/javascript">

    function submithelper() {

      // this is why we need the id="xxx" below.
      var $domainInput = document.getElementById("domain");
      var $extInput = document.getElementById("ext");

      var $domain = $domainInput.value;
      var $i = $domain.indexOf('.'); 
      var $host = $domain.substr(0, $i); // perhaps skip to do this if it isn't used?
      var $ext = $domain.substr($i);          

      $extInput.value = $ext;

      return true; // false to stop form from submitting.
    }

</script>

<form onsubmit="return submithelper()" method="post" action="https://www.jms-networks.net/domainchecker.php">

  <!-- Two inputs has been given IDs so we can easily get them from javascript. -->

  <span class="text"><input id="domain" value="{php} print $_POST['sld'] ? $_POST['sld'] : 'Search for your domain name'; {/php}" type="text" name="domain" class="searchbox" onfocus="if (this.value == this.defaultValue) this.value = '';" onblur="if (this.value == '') this.value = this.defaultValue;" /></span>
  <input type="hidden" name="direct" value="true" />
  <input id="ext" type="hidden" name="tlds[]" value="" />
  ...