add information

Author: shuangela

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

@@ -38,7 +38,9 @@ you can use regular expressions:
 
 The regular expression pattern you pass into a filter must follow the :query:`regex <$regex>` syntax supported by the MongoDB server.
 
-Regular expressions in filter documents use the following ``options``:
+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``.
 
 .. list-table::
    :header-rows: 1
@@ -141,8 +143,8 @@ creating a series of filters for individual databases or groups of collections.
 Details
 =======
 
-Regular Expression Options
---------------------------
+Regular Expression Options Example
+----------------------------------
 
 ``databaseRegex`` and ``collectionsRegex`` each supports an ``options`` field,
 which you can use to configure regular expression options.
@@ -151,7 +153,10 @@ Internally, ``mongosync`` passes the filter and options to the
 with Filtred Sync.
 
 For example, this filter would match collections in the ``sales`` database
-that begin start with the ``accounts_`` string:
+that begin with the ``accounts_`` string. The filter also specifies the option ``m`` to
+match characters at the beginning or end of each line for strings with multiline values, and the
+option ``s`` to allow the dot character to match all characters including newline
+characters.
 
 .. code-block:: json