The QRegularExpression rx variable inside the implementation of QSortFilterProxyModel::setFilterRegularExpression(const QString &) is unused. QRegularExpression::setPattern() leaves the pattern options unchanged. The intent of the current code seems to be: reset all pattern options except for QRegularExpression::CaseInsensitiveOption to defaults. This can be achieved by removing the unused variable rx and adding the following line:

d->filter_data.setPatternOptions(d->filter_data.patternOptions() & QRegularExpression::CaseInsensitiveOption);

Before Qt 5.15.2 this member function preserved none of the pattern options. With https://codereview.qt-project.org/c/qt/qtbase/+/310989 included in Qt 5.15.2, only QRegularExpression::CaseInsensitiveOption is preserved.