We can get you partially there. The GUI does not allow conversions with that level of specificity. We do support client side scripts to run prior to the form submission.
Edit the Register Page Form fields and select submit and Edit. Under Advanced you will see an option AJAX Submit. Enter: RegisterSubmit and Save Changes.
Return to the edit self-registration map and select the Register Page Footer. In the Footer HTML textarea we need to define the function that will morph your field.
Note that a scripting language is available on these fields, so any CSS or JavaScript needs to be wrapped in special tags:
{literal} .. {/literal}
The following example was a proof of concept that reversed a phone number, stripping any extraneous characters.
<script type="text/JavaScript"><!--{literal}
function RegisterSubmit(form_name) {
var frm = document.forms[form_name];
var error = '';
// Checking here...
if (error != '') {
alert(error);
Nwa_EnableFormButtons(form_name);
return false;
}
// Reverse the phone number and put it as the username.
if (frm.visitor_phone.value != '') {
var name = '';
var phone = frm.visitor_phone.value.split('');
for (var i = 0; i < phone.length; i++) {
var charCode = phone[i].charCodeAt(0);
if((charCode > 47) && (charCode < 58)) {
name = phone[i] + name;
}
}
frm.username.value = name;
}
return true; // Proceed to regular
}
//-->
{/literal}</script>
In your case you would want to pull in your sponsor_email field, break apart and piece it back together formatted as you need.
These may assist in your specific needs:
http://lawrence.ecorp.net/inet/samples/regexp-format.php#cap
http://www.w3schools.com/jsref/jsref_split.asp
http://www.sitepoint.com/forums/showthread.php?706553-Need-to-split-email-address-and-have-two-alert-outputs