I really like AjaxSearch - it has saved me a LOT of time. Here is a little mod I did to give an alert message if the search string is too short.
Add this:
if(s.length < 4) {
alert('Search string must be at least 4 characters.');
return false;
}
after
// Same if the search is blank
if (s == '') return false;
in AjaxSearch.js function doSearch().
