list.observe('click', function(event){
  var btn = event.findElement('a._ip');
  if(btn && (btn_class = btn.className.match(new RegExp("\\b_ip_btn_\\S+", "g")))){
     switch(btn_class.first()) {
       case '_ip_btn_filter': //do_something;
         break;
       case '_ip_btn_edit': //do something;
	 break;
       case '_ip_btn_destroy':	//do something;
	 break;
       default: // default
    };
  };
});

alternative:

list.observe('click', function(event){
  var btn = event.findElement('a._ip_bla_bla');
  if(btn)
     //do_something;
  var btn = event.findElement('a._ip_hu_hu');
    if(btn)
     //do_something;
  var btn = event.findElement('a._ip_tam_tam');
    if(btn)
     //do_something;
});