1. Clone qt5.git:

   git clone -b v6.8.1 git://code.qt.io/qt/qt5.git
   

2. Initialize qtbase:

   cd qt5/
   perl init-repository --module-subset=qtbase
   

3. Try configuring Qt in a directory named after the used compiler:

   mkdir ../g++
   cd !$
   ../qt5/configure -release -prefix $PWD/qtbase -opensource -confirm-license
   

   That fails:

   -- [QtBase] CMAKE_BUILD_TYPE was already explicitly set to: 'Release'
   RegularExpression::compile(): Nested *?+.
   RegularExpression::compile(): Error in compile.
   CMake Error at qtbase/cmake/QtPublicSbomHelpers.cmake:1531 (elseif):
     given arguments:
   
       "path" "MATCHES" "^/tmp/qt/g++/qtbase/"
   
     Regular expression "^/tmp/qt/g++/qtbase/" cannot compile
   

This used to work correctly in Qt 6.8.0.