Email thread:

  https://sourceforge.net/p/roundup/mailman/roundup-
users/thread/MWHPR08MB2941242ADD82F47F14E7FCAFCA7B0%40MWHPR08MB2941.namprd08.prod.outl
ook.com/#msg37065950

with subject "Trouble with new tracker designator?" on roundup users.

TL;DR

We need to restrict the format of classnames passed to the Class() method of the
hyperdb to --

  Alphabetic, one or more word char (alphabetic, numeric and _) and
  ending with an alphabetic or _

(Note classname can't easily end in a number as that makes it costly to
split the item number from the class name in a designator.)

Also modify splitDesignator() to match the classname format above and reject
all others.

Add tests for these cases.

Also need to find a place to document the class name restrictions. The design
doc only says has to start with an alphabetic.

This may drive a rewrite of the customizing doc to separate out API/internals 
documentation from actual customization examples. The current doc has grown
and it's tough to find things in there.

Do not allow leading _ at this time. We reserve it for future use if other
issues (e.g. the database tables names for user defined class names have
an _ prepended to the user class name) don't prevent it from being used.