Richard Stallman <rms@gnu.org> writes:

>     ! 	  (unless (save-match-data
>     ! 		    (save-excursion
>     ! 		      (goto-char (match-beginning 0))
>     ! 		      (string-match
>     ! 		       (format "^\t* \\{0,%d\\}$"
>     ! 			       (- tab-width (1+ (mod (current-column) tab-width))))
>     ! 		       (match-string 0))))
>
> That would be very slow.  Please do it without using regexps,
> perhaps using skip-chars-forward.

I can come up with the following regexp-free, search-free patch that
makes the job, and that uses Stefan's good proposal of using " [ \t]+"
as `tabify-regexp'.

However, the code now supposes that if a TAB appears in the match,
a space must have preceded it.  I specified that in the docstring.