add info

Author: shuangela

source/reference/collection-level-filtering/filter-regex.txt

@@ -40,7 +40,8 @@ The regular expression pattern you pass into a filter must follow the :query:`re
 
 Regular expressions in filter documents use ``options`` listed in the :query:`regex
 <$regex>` guide. ``options`` is a string of concatenated options. For example, to specify
-the ``i`` and ``s`` options, pass in "si" to ``options``.
+the ``i`` and ``s`` options, pass in "si" to ``options``. The order of concatenated
+options does not matter.
 
 .. list-table::
    :header-rows: 1