Help with javacript field check

How do I check an entry field in my form called affiliatecode to make sure it has the string "register.i-say.com" somewhere in it?

 

This js (external file) may help point you in the right direction - see the last part where it checks for an @ in the email address:

function validateForm(theform) {
 for (var i=0; i<theform.elements.length; i++) {
  var element = theform.elements[i];
  if (element.className.indexOf("required") !=-1) {
   element.className = "required";
   if (!isFilled(element)) {
    alert("Please enter your " +element.id);
    element.className += " input";
    element.focus();
    return false;
   } 
  }
  if (element.className.indexOf("email") !=-1) {
   element.className = "email";
   if (!isEmail(element)) {
    alert("Please check you have entered a valid email address");
    element.className += " input";
    element.focus();
    return false;
   }
  }
 }
 return true;
}

function isFilled(field) {
 if (field.value.length < 1) {
  return false;
 } else {
  return true;
 }
}
function isEmail(field) {
 if (field.value.indexOf("@") == -1 || field.value.indexOf(".") == -1) {
  return false;
 } else {
  return true;
 }
}

Code (markup):