[ExternalData] ExternalData_Expand_Arguments with REGEX arguments - how to get matched files at configure time?

Usage
1

When I issue the command ExternalData_Expand_Arguments with a REGEX argument to match multiple files, I was expecting to get all matched files as a return argument, but it returns the directory on which the REGEX operates. Of course, the ExternalData module is aware of all files matched, because during make MY_DATA these files are downloaded correctly.

How do I get the list of matched files at configure time?

ExternalData_Expand_Arguments(
  MY_DATA, 
  _dataFiles, 
  DATA{${CMAKE_CURRENT_SOURCE_DIR}/grids/,REGEX:.*cgns}}
)
message("_dataFiles => ${_dataFiles}")

and the output is

_dataFiles => /path/to/current/source/dir/grids

I expected the following

_dataFiles => /path/to/current/source/dir/grids/grid1.cgns;/path/to/current/source/dir/grids/grid2.cgns;/path/to/current/source/dir/grids/grid3.cgns
2

After inspection of the ExternalData module, I found that the list of files can be found in the global properties _ExternalData_MY_DATA_FETCH and _ExternalData_MY_DATA_LOCAL. The latter property has no content, whereas the former property contains the list of files.