bpo-31672: string: Use re.A | re.I flag for identifier pattern

Doc/library/string.rst

@@ -755,8 +755,18 @@ 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]*)``. Since default *flags* is
+ ``re.IGNORECASE``, ``[a-z]``Without local flag ``-i``, is used to avoid to match with non ASCII characters.
+ If this is given and *braceidpattern* is
+ ``None`` this pattern will also apply to braced placeholders.
+
+ .. note::
+
+ Default *flags* is ``re.IGNORECASE``. So the pattern ``[a-z]`` can match
+ with some non ASCII characters. That's why We use local ``-i`` flag here.
+
+ When overrinding this class, please consider overriding *flags* with ``0``
+ or ``re.IGNORECASE | re.ASCII``.
 
  .. versionchanged:: 3.7
  *braceidpattern* can be used to define separate patterns used inside and

Lib/string.py

@@ -79,7 +79,11 @@ class Template(metaclass=_TemplateMetaclass):
  """A string class for supporting $-substitutions."""
 
  delimiter = '$'
- idpattern = r'[_a-z][_a-z0-9]*'
+ # 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.
+ # See https://bugs.python.org/issue31672
+ idpattern = r'(?-i:[_a-zA-Z][_a-zA-Z0-9]*)'
  braceidpattern = None
  flags = _re.IGNORECASE
 

Lib/test/test_string.py

@@ -270,6 +270,10 @@ def test_invalid_placeholders(self):
  raises(ValueError, s.substitute, dict(who='tim'))
  s = Template('$who likes $100')
  raises(ValueError, s.substitute, dict(who='tim'))
+ # Template.idpattern should match to only ASCII characters.
+ # https://bugs.python.org/issue31672
+ s = Template("$who likes $ı") # (0x131, DOTLESS I)
+ raises(ValueError, s.substitute, dict(who='tim'))
 
  def test_idpattern_override(self):
  class PathPattern(Template):

Misc/NEWS.d/next/Library/2017-10-12-02-47-16.bpo-31672.DaOkVd.rst

@@ -0,0 +1,2 @@
+``idpattern`` in ``string.Template`` matched some non ASCII characters. Now
+it uses ``-i`` regular expression local flag to avoid non ASCII characters.