Thanks, All. But I think spirit is basic right. It has nothing to do with if the RegExp is correct. The real problem is hierarchical data model. I need to subclass QSortFilterProxyModel to do something concerning "source_parent":

Qt Code: 
  1. class MySortFilterProxyModel : public QSortFilterProxyModel
  2. {
  3. protected:
  4.     virtual bool MySortFilterProxyModel::filterAcceptsRow(
  5.             int source_row, const QModelIndex &source_parent) const{
  6.         QFileSystemModel *sm = qobject_cast<QFileSystemModel*>(sourceModel());
  7.         if (source_parent == sm->index(sm->rootPath())) {    
  8.             return QSortFilterProxyModel::filterAcceptsRow(source_row, source_parent);
  9.         } 
  10.         return true;            
  11.     }
  12. };
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This works fine for me.