function validate(form_id,email_address) {
   var reg = /^([A-Za-z0-9_\-\.])+\@([A-Za-z0-9_\-\.])+\.([A-Za-z]{2,4})$/;
   var address = document.forms[form_id].elements[email_address].value;
   if(reg.test(address) == false) {
      alert('Please enter a valid email address.');
      return false;
   }
}
<script type="text/javascript" src="http://w.sharethis.com/button/buttons.js"></script>
stLight.options({publisher:'24c3625c-fc4b-43fb-9115-dab65ecc2125'});

function clearDefault(el) {
  if (el.defaultValue==el.value) el.value = ""
}