Re: custom type `color' is not enforced

[Lennart Borgman (gmail)]

Richard Stallman wrote:
> I agree that these changes would be improvements.  However, I don't
> think we have any way to test the validity of a regexp without
> matching it, and matching it (even against a null string) could
> cause an infinite loop.

The widget library already validates

  :type 'regexp

with the function (defun widget-regexp-validate (widget) ... ) which 
does (string-match regexp "").