I would like to begin this e-mail noting that the word "qualifier" is used
in the specification for two very different things: season qualifiers (as
of #209) and Uncertain / Approximate (as of #103). I suggest to choose
more consistent formulations.
Defining qualifier strings is quite challenging because it should cover
any written language (including languages who need the PUA) and also be
easily parsable and included within different serialized formats. It
should also be forward compatible to allow for extensions within phase
two, which may (perhaps) include namespaces. It should also be easily
included in XML attributes or JSON, for example. Furthermore, this forward
compatibility show an awareness that we may need to define qualifiers for
more than just seasons.
I have been thinking about this issue and how to write something useful
covering the above requirements in most cases. Now, I suggest the
following regex to be used for the production of qualifier strings: