Re: [Proposal] M-x tabify to indent only when needed.

[Richard Stallman]

    +          (if (= (/ end-col tab-width) (/ beg-col tab-width))
    +              ;; The spacing does not straddle a TAB boundary, so we won't
    +              ;; be able to use a TAB here anyway: there's nothing to do.
    +              nil

This detects a case where no tabs can be used.
It doesn't detect the case where tabs already ARE used.

    -(defvar tabify-regexp "[ \t][ \t]+"
    +(defvar tabify-regexp " [ \t]+"

Is that change supposed to exclude those other cases?
It is a good idea to make the regexp match less often.