gistfile1.txt

trigger ApplyAssignmentRules on Lead (after insert) {
 
  List<User> owner = [Select u.Id From User u where Name = 'User Portal' limit 1];
  
	if( owner.isEmpty() )
		return;

  List<Lead> LeadsToUpdate = new List<Lead>();
  
  For (Lead l:trigger.new)
  	if (l.OwnerId == owner.get(0).Id)
  		LeadsToUpdate.add(l);

  if( LeadsToUpdate.isEmpty() )
      return;

  Database.DMLOptions dmo = new Database.DMLOptions();
  dmo.assignmentRuleHeader.useDefaultRule= true;
  
  For (Lead u:LeadsToUpdate)
  	u.setOptions( dmo );

  update LeadsToUpdate;
}