I think, the best and flexible way is to make the checking by JavaScript.
For example, open templates/user_dataform.html for editing and edit the following:
1) Between <script></script> code, before //--> insert new JavaScript function:
function checkFields(){
var error=false;
if(document.forms['regform'].from.value=='') error=true;
else if(document.forms['regform'].occupation.value=='') error=true;
if(error) alert ('Please, fill in all necessary fields.');
else document.forms['regform'].submit();
}
2) Replace input type=submit in a form by input type=button onClick="JavaScript:checkFields();" |