Message 100861 - Python tracker

I'm not sure I'm wild about doing the work twice, once as string and once as unicode if need be. But I'll consider it, especially since this is only a 2.7 issue.

There could be side effects of evaluating the replacement strings, but I'm not sure it's worth worrying about. Attribute (or index) access having side effects isn't something I think we need to cater to.