The test for re.engine was misfiring because re.engine is no longer
defined and the default was "pre" instead of "sre".  Give up on 1.5.2
compatibility, hardcode the sre solution.  However, this XXX comment
still applies, AFAIK:
        # XXX This code depends on internals of the regular expression
        # engine!  There's no standard API to do a substitution when you
        # have already found the match.  One should be added.