When calling QSortFilterProxyModel::setCaseSensitivity the condition for QRegularExpression is inverted:

options.setFlag(QRegularExpression::CaseInsensitiveOption, cs == Qt::CaseSensitive);

Also calling QSortFilterProxyModel::setFilterRegularExpression(const QString &pattern) ignores QSortFilterProxyModel::caseSensitivity, while QSortFilterProxyModel::setFilterRegExp does take it into account.

The documentation for both says

Setting this property overwrites the current filterCaseSensitivity.

But I guess it mainly refers to the overloads taking a QRegExp/QRegularExpression since they already have a CaseSensitivity state, while a string doesn't.