bpo-31672: Fix string.Template accidentally matched non-ASCII identifiers (GH-3872)

Pattern `[a-z]` with `IGNORECASE` flag can match to some non-ASCII characters.

Straightforward solution for this is using `IGNORECASE | ASCII` flag.
But users may subclass `Template` and override only `idpattern`. So we want to
avoid changing `Template.flags`.

So this commit uses local flag `-i` for `idpattern` and change `[a-z]` to `[a-zA-Z]`.
(cherry picked from commit b22273ec5d1992b0cbe078b887427ae9977dfb78)

1 files changed, 12 insertions, 2 deletions

diff --git a/Doc/library/string.rst b/Doc/library/string.rst
index a0977b6..7a9fcc3 100644
--- a/Doc/library/string.rst
+++ b/Doc/library/string.rst
@@ -746,8 +746,18 @@ to parse template strings.  To do this, you can override these class attributes:
 
 * *idpattern* -- This is the regular expression describing the pattern for
   non-braced placeholders (the braces will be added automatically as
-  appropriate).  The default value is the regular expression
-  ``[_a-z][_a-z0-9]*``.
+  appropriate). The default value is the regular expression
+  ``(?-i:[_a-zA-Z][_a-zA-Z0-9]*)``.
+
+  .. 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.
+
+     While *flags* is kept to ``re.IGNORECASE`` for backward compatibility,
+     you can override it to ``0`` or ``re.IGNORECASE | re.ASCII`` when
+     subclassing.
+
 
 * *flags* -- The regular expression flags that will be applied when compiling
   the regular expression used for recognizing substitutions.  The default value