in such a list is removed from the associated
  main list, which in this example, would be "sources".  Removed items are
  placed into a "sources_excluded" list in the dict.

  Regular expression (regex) filters are contained in dict keys named with a
  trailing "/", such as "sources/" to operate on the "sources" list.  Regex
  filters in a dict take the form:
    'sources/': [ ['exclude', '_(linux|mac|win)\.cc$'],
                  ['include', '_mac\.cc$'] ],
  The first filter says to exclude all files ending in _linux.cc, _mac.cc, and
  _win.cc.  The second filter then includes all files ending in _mac.cc that
  are now or were once in the "sources" list.  Items matching an "exclude"
  filter are subject to the same processing as would occur if they were listed
  by name in an exclusion list (ending in "!").  Items matching an "include"
  filter are brought back into the main list if previously excluded by an
  exclusion list or exclusion regex filter.  Subsequent matching "exclude"
  patterns can still cause items to be excluded after matching an "include".
  r