Mono (and the .NET documentation) requires that a named backreference be
specified as \k<name> rather than just \<name>
Mono treats the last file.ext as being the filename when deciding what the
PathInfo is (rather than the first); resulting in blank PathInfo
for /cassette.axd/file/wherever.png

This can be trivially worked around by using request.RAwUrl instead
More \k<name> backreference fixes