[XeTeX] wildcard interchar class?

[Michiel Kamermans]

Hi all,

would it be hard to add a "wildcard" intercharclass to XeTeX, to allow 
signalling 'going to' and 'leaving' character classes? I was thinking 
along the lines of a special class lets you say:

\XeTeXinterchartoks * 4 = {\classstartcommand{something}{or}{other}}
\XeTeXinterchartoks 4 * = {\classendcommand{something}{or}{other}}

This would simplify those setups where the change behaviour is dictated 
not so much by which transition occurs, but by which class is 
encountered, and would remove the need for adding transition rules for 
every class already in the preamble when a new character class gets added.

In terms of precedence, the specification order could be used to 
determine transition behaviour, with the wildcard binding pragmatically 
needing to be listed first, followed by specific inter-class transition 
rules, so that the two never conflict: either a transition is caught by 
two wildcard rules (the end of the current class, and the start of the 
next), or there's an explicit rule that overrides the default behaviour.

I don't know how hard it would be to add this, but this functionality 
would seem like a desirable feature to have in XeTeX because it doesn't 
interfere with the way things work right now, but would greatly reduce 
the need for transition rules whens only start/end rules are needed; 
something which even for the five default classes (0, 1, 2, 3, 255) 
would reduce the number of required transition rules from 8 per class 
(increasing linear as 2n-2, with total rule count being the polynomial 
n(2n-2)) to only 2 (a fixed number irrespective of class count, with 
total rule count being the linear 2n).

- Mike
nihongoresources.com