diff --git a/Lib/re.py b/Lib/re.py
index abd7ea2..cdf5976 100644
--- a/Lib/re.py
+++ b/Lib/re.py
@@ -215,12 +215,14 @@
     return _compile(pattern, flags|T)
 
 _alphanum_str = frozenset(
-    "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ01234567890")
+    "_abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ01234567890")
 _alphanum_bytes = frozenset(
-    b"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ01234567890")
+    b"_abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ01234567890")
 
 def escape(pattern):
-    "Escape all non-alphanumeric characters in pattern."
+    """
+    Escape all the characters in pattern except ASCII letters, numbers and '_'.
+    """
     if isinstance(pattern, str):
         alphanum = _alphanum_str
         s = list(pattern)
diff --git a/Lib/test/test_re.py b/Lib/test/test_re.py
index fe8bc34..e3d10f7 100644
--- a/Lib/test/test_re.py
+++ b/Lib/test/test_re.py
@@ -428,7 +428,7 @@
         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 = ''.join(chr(i) for i in range(256))
         for c in p:
             if c in alnum_chars:
@@ -441,7 +441,7 @@
         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 = bytes(range(256))
         for i in p:
             b = bytes([i])