bpo-1198569: Allow string.Template braced pattern to be different (#3288)

* bpo-1198569: Allow the braced pattern to be different

``string.Template`` subclasses can optionally define ``braceidpattern`` if
they want to specify different placeholder patterns inside and outside the
braces.  If None (the default) it falls back to ``idpattern``.

1 files changed, 15 insertions, 3 deletions

diff --git a/Doc/library/string.rst b/Doc/library/string.rst
index 8176a81..1a9b630 100644
--- a/Doc/library/string.rst
+++ b/Doc/library/string.rst
@@ -754,9 +754,21 @@ attributes:
   be set in the subclass's class namespace).
 
 * *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]*``.
+  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.
+
+  .. versionchanged:: 3.7
+     *braceidpattern* can be used to define separate patterns used inside and
+     outside the braces.
+
+* *braceidpattern* -- This is like *idpattern* but describes the pattern for
+  braced placeholders.  Defaults to ``None`` which means to fall back to
+  *idpattern* (i.e. the same pattern is used both inside and outside braces).
+  If given, this allows you to define different patterns for braced and
+  unbraced placeholders.
+
+  .. versionadded:: 3.7
 
 * *flags* -- The regular expression flags that will be applied when compiling
   the regular expression used for recognizing substitutions.  The default value