proposal: defconst/defcustom org-tags-regexp

[Adam Spiers <orgmode@adamspiers.org>]

Hi again,

There seem to be a number of hardcoded regexps currently used for
matching heading tags, all very similar looking, and typically
something like:

  [ \t]*\\(:[[:alnum:]_@:]+:\\)?[ \t]*\\($\\|\r\\)

Is there any reason why these shouldn't be factored out into a new
defcustom org-tags-regexp?  Not only would this make for more reliable
code as per the DRY principle, but it would also some new
possibilities:

It seems a lot of people are already using "@" as a tag prefix to
indicate GTD contexts, which the code currently allows, and is a nice
way of keeping distinct tag "namespaces".  Additionally I would like
to have tags to denote time estimates for actions:

     <10min    i.e. it's a 10 minute job worst case
     <30min    i.e. it's a half an hour job worst case
     <60min    etc.
     <120min
     <4hrs
     <1day

(currently, the "<" prefix confuses the code); and maybe something
else, like "+" to prefix tags which denote actions forming part of a
bigger picture e.g. "+health" for any actions/projects related to
living more healthily (less elisp hacking and more sunlight might be a
good start ;-)

Thanks,
Adam