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)