I think it's a mistake for Template.safe_substitute()
to raise a ValueError when the invalid group is
matched.  I just ran across such a situation in Real
Code that is using Template.  In my code, I'm using a
subclass to override idpattern and then calling
safe_substitute().  The source of the template is a
flattened mail message where what I'm substituting is
going into some of the header fields.

The body of the message contains a big blurb of XML,
including the string "...$1...".  Well, that $1
triggers the ValueError.

This patch changes the semantics of safe_substitute()
so that when the 'invalid' group matches, the original
delimiter is returned, which I think is the right thing
to do.  Included is an updated test (I will update the
docs too if you agree that this patch should be applied).