REGEX expression in install command is converted to lower case

CMake 3.8.1, MacOS 10.12 When specifying a REGEX expression to EXCLUDE files to be installed with INSTALL(DIRECTORY)the expression gets converted to lowercase. The following statement in a CMakeLists.txt file

install(DIRECTORY mydir DESTINATION include REGEX aAbBcC EXCLUDE)

produces the cmake_install.cmake file with

if("${CMAKE_INSTALL_COMPONENT}" STREQUAL "Unspecified" OR NOT CMAKE_INSTALL_COMPONENT)
  file(INSTALL DESTINATION "${CMAKE_INSTALL_PREFIX}/include" TYPE DIRECTORY FILES "/Users/mato/mydir" REGEX "aabbcc" EXCLUDE)
endif()