diff -r 062533327ad2 Lib/re.py
--- a/Lib/re.py	Mon Aug 19 10:03:25 2013 +0300
+++ b/Lib/re.py	Mon Aug 19 22:54:23 2013 +0800
@@ -199,10 +199,11 @@
     return _compile(pattern, flags|T)
 
 _alphanum = frozenset(
-    "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789")
+    "_abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789")
 
 def escape(pattern):
-    "Escape all non-alphanumeric characters in pattern."
+    """Escape all non-alphanumeric characters and non-underscore characters
+    in pattern."""
     s = list(pattern)
     alphanum = _alphanum
     for i, c in enumerate(pattern):
diff -r 062533327ad2 Lib/test/test_re.py
--- a/Lib/test/test_re.py	Mon Aug 19 10:03:25 2013 +0300
+++ b/Lib/test/test_re.py	Mon Aug 19 22:54:23 2013 +0800
@@ -501,7 +501,7 @@
         self.assertEqual(m.span(), span)
 
     def test_re_escape(self):
-        alnum_chars = string.ascii_letters + string.digits
+        alnum_chars = string.ascii_letters + string.digits + '_'
         p = u''.join(unichr(i) for i in range(256))
         for c in p:
             if c in alnum_chars:
@@ -514,7 +514,8 @@
         self.assertMatch(re.escape(p), p)
 
     def test_re_escape_byte(self):
-        alnum_chars = (string.ascii_letters + string.digits).encode('ascii')
+        alnum_chars = (string.ascii_letters + string.digits + '_').\
+            encode('ascii')
         p = ''.join(chr(i) for i in range(256))
         for b in p:
             if b in alnum_chars: