add regex info

Author: shuangela

source/reference/collection-level-filtering.txt

@@ -85,6 +85,9 @@ Filters have the following syntax:
        }
     ]
 
+To learn more about syntax for ``regex-pattern`` and options for regular
+expression filtering, see :ref:`c2c-filter-regex`. 
+
 Filters must include either the ``database`` field or the ``databaseRegex`` field.
 
 If you need the filter to match specific collections, you can use either 

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

@@ -36,6 +36,63 @@ you can use regular expressions:
       }
    }
 
+The regular express pattern you pass into a filter must follow the :query:`regular
+expression <$regex>` syntax supported by the MongoDB server.
+
+The following ``options`` are available for use with regular
+expressions:
+
+.. list-table::
+   :header-rows: 1
+   :widths: 20 80
+
+   * - Option
+     - Description
+
+   * - ``i``
+     - Case insensitivity to match upper and lower cases. For an
+       example, see :ref:`regex-case-insensitive`.
+
+   * - ``m``
+
+     - For patterns that include anchors (i.e. ``^`` for the start,
+       ``$`` for the end), match at the beginning or end of each
+       line for strings with multiline values. Without this option,
+       these anchors match at beginning or end of the string. For an
+       example, see :ref:`regex-multiline-match`.
+
+       If the pattern contains no anchors or if the string value has
+       no newline characters (e.g. ``\n``), the ``m`` option has no
+       effect.
+
+   * - ``x``
+
+     - "Extended" capability to ignore all white space characters in
+       the ``$regex`` pattern unless escaped or included in a
+       character class.
+
+       Additionally, it ignores characters in-between and including
+       an un-escaped hash/pound (``#``) character and the next new
+       line, so that you may include comments in complicated
+       patterns. This only applies to data characters; white space
+       characters may never appear within special character
+       sequences in a pattern.
+
+       The ``x`` option does not affect the handling of the VT
+       character (i.e. code 11).
+
+   * - ``s``
+
+     - Allows the dot character (i.e. ``.``) to match all
+       characters *including* newline characters. For an example,
+       see :ref:`regex-dot-new-line`.
+
+   * - ``u``
+
+     -  Supports Unicode. This flag is accepted, but is redundant. UTF is set by 
+        default in the ``$regex`` operator, making the ``u`` option 
+        unnecessary. 
+
 Regular expressions in filter documents use the following fields:
 
 .. list-table::