bpo-31672 - Add one last minor clarification for idpattern (#4483)

Add one last minor clarification for idpattern

Doc/library/string.rst

@@ -755,13 +755,15 @@ attributes:
 
 * *idpattern* -- This is the regular expression describing the pattern for
   non-braced placeholders.  The default value is the regular expression
-  ``(?-i:[_a-zA-Z][_a-zA-Z0-9]*)``.  If this is given and *braceidpattern* is
+  ``(?a:[_a-zA-Z][_a-zA-Z0-9]*)``.  If this is given and *braceidpattern* is
   ``None`` this pattern will also apply to braced placeholders.
 
   .. note::
 
      Since default *flags* is ``re.IGNORECASE``, pattern ``[a-z]`` can match
-     with some non-ASCII characters. That's why we use local ``-i`` flag here.
+     with some non-ASCII characters. That's why we use the local ``a`` flag
+     here.  Further, with the default *flags* value, including ``A-Z`` in the
+     ranges is redundant, but required for backward compatibility.
 
      While *flags* is kept to ``re.IGNORECASE`` for backward compatibility,
      you can override it to ``0`` or ``re.IGNORECASE | re.ASCII`` when

Lib/string.py

@@ -79,11 +79,14 @@ class Template(metaclass=_TemplateMetaclass):
     """A string class for supporting $-substitutions."""
 
     delimiter = '$'
-    # r'[a-z]' matches to non-ASCII letters when used with IGNORECASE,
-    # but without ASCII flag.  We can't add re.ASCII to flags because of
-    # backward compatibility.  So we use local -i flag and [a-zA-Z] pattern.
+    # r'[a-z]' matches to non-ASCII letters when used with IGNORECASE, but
+    # without the ASCII flag.  We can't add re.ASCII to flags because of
+    # backward compatibility.  So we use the ?a local flag and [a-z] pattern.
+    # We also can't remove the A-Z ranges, because although they are
+    # technically redundant with the IGNORECASE flag, the value is part of the
+    # publicly documented API.
     # See https://bugs.python.org/issue31672
-    idpattern = r'(?-i:[_a-zA-Z][_a-zA-Z0-9]*)'
+    idpattern = r'(?a:[_a-zA-Z][_a-zA-Z0-9]*)'
     braceidpattern = None
     flags = _re.IGNORECASE