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]`.

1 files changed, 11 insertions, 2 deletions

diff --git a/Doc/library/string.rst b/Doc/library/string.rst
index 1a9b630975..1076cdb234 100644
--- a/Doc/library/string.rst
+++ b/Doc/library/string.rst
@@ -755,8 +755,17 @@ attributes:
 
 * *idpattern* -- This is the regular expression describing the pattern for
   non-braced placeholders.  The default value is the regular expression
-  ``[_a-z][_a-z0-9]*``.  If this is given and *braceidpattern* is ``None``
-  this pattern will also apply to braced placeholders.
+  ``(?-i:[_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.
+
+     While *flags* is kept to ``re.IGNORECASE`` for backward compatibility,
+     you can override it to ``0`` or ``re.IGNORECASE | re.ASCII`` when
+     subclassing.  It's simple way to avoid unexpected match like above example.
 
   .. versionchanged:: 3.7
      *braceidpattern* can be used to define separate patterns used inside and