The regular expressions in string.Template can be
overidden, but there's not an easy way to override the
expression for variables inside braces and to a
different expression than variables not in braces. 
E.g., you might allow full expressions in braces
(including arbitrary punctuation), but only normal
variable names outside.  An example of using this kind
of technique:

http://svn.colorstudy.com/home/ianb/recipes/sub_expr.py

So, in summary, idpattern should be split into two,
idpattern and bracepattern.  (Maybe a default of None
for bracepattern, and use idpattern if bracepattern is
None -- this would be backward compatible so that
overriding only idpattern will effect both)

IIUC, Ian's link has a doctest for this feature.

This seems like a reasonable request.  Do you care to submit a patch with tests and doc updates?

Sorry for the long delay.  It looks like Ian's original link is no longer valid.  Do you have an updated link to a possible doctest or example?

Never mind; I crafted a decent test for the PR.

New changeset ba4279683f8eb8f59be10d12547ea89480614388 by Barry Warsaw in branch 'master':
bpo-1198569: Allow string.Template braced pattern to be different (#3288)
https://github.com/python/cpython/commit/ba4279683f8eb8f59be10d12547ea89480614388

New changeset 973b901212bd84d279904bab6654709f4ec32470 by Barry Warsaw in branch 'master':
What's New for bpo-1198569 (#3303)
https://github.com/python/cpython/commit/973b901212bd84d279904bab6654709f4ec32470