#2650: re.escape() no longer escapes the "_".

Doc/library/re.rst

@@ -689,9 +689,12 @@ form.
 
 .. function:: escape(string)
 
-   Return *string* with all non-alphanumerics backslashed; this is useful if you
-   want to match an arbitrary literal string that may have regular expression
-   metacharacters in it.
+   Escape all the characters in pattern except ASCII letters, numbers and ``'_'``.
+   This is useful if you want to match an arbitrary literal string that may
+   have regular expression metacharacters in it.
+
+   .. versionchanged:: 3.3
+      The ``'_'`` character is no longer escaped.
 
 
 .. function:: purge()

Lib/re.py

@@ -215,12 +215,14 @@ def template(pattern, flags=0):
     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)

Lib/test/test_re.py

@@ -428,7 +428,7 @@ class ReTests(unittest.TestCase):
         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 @@ class ReTests(unittest.TestCase):
         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])

Misc/NEWS

@@ -98,6 +98,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #2650: re.escape() no longer escapes the '_'.
+
 - Issue #11757: select.select() now raises ValueError when a negative timeout
   is passed (previously, a select.error with EINVAL would be raised).  Patch
   by Charles-François Natali.