Simplified test and added what's new note.

Author: mbrucher

doc/source/whatsnew/v0.18.2.txt

@@ -493,6 +493,7 @@ Bug Fixes
 - Bug in ``pd.read_csv()`` in which the ``nrows`` argument was not properly validated for both engines (:issue:`10476`)
 - Bug in ``pd.read_csv()`` with ``engine='python'`` in which infinities of mixed-case forms were not being interpreted properly (:issue:`13274`)
 - Bug in ``pd.read_csv()`` with ``engine='python'`` in which trailing ``NaN`` values were not being parsed (:issue:`13320`)
+- Bug in ``pd.read_csv()`` with ``engine='python'`` when reading from a tempfile.TemporaryFile on Windows with Python 3 a file with the separator expressed as a regex (:issue:`13398`)
 - Bug in ``pd.read_csv()`` that prevents ``usecols`` kwarg from accepting single-byte unicode strings (:issue:`13219`)
 - Bug in ``pd.read_csv()`` that prevents ``usecols`` from being an empty set (:issue:`13402`)
 - Bug in ``pd.read_csv()`` with ``engine=='c'`` in which null ``quotechar`` was not accepted even though ``quoting`` was specified as ``None`` (:issue:`13411`)

pandas/io/tests/parser/python_parser_only.py

@@ -174,14 +174,7 @@ def test_read_table_buglet_4x_multiindex(self):
 
     def test_temporary_file(self):
         # GH13398
-        data1 = """index,A,B,C,D
-foo,2,3,4,5
-bar,7,8,9,10
-baz,12,13,14,15
-qux,12,13,14,15
-foo2,12,13,14,15
-bar2,12,13,14,15
-"""
+        data1 = "0,0"
         data2 = data1.replace(",", " ")
 
         from tempfile import TemporaryFile
@@ -190,6 +183,7 @@ def test_temporary_file(self):
         new_file.flush()
         new_file.seek(0)
 
-        result = self.read_csv(new_file, sep=r"\s+", engine="python")
-        expected = self.read_csv(StringIO(data1))
+        result = self.read_csv(new_file, sep=r"\s+", header=None)
+        expected = DataFrame([[0, 0]])
         tm.assert_frame_equal(result, expected)
+