tabify
tabify is an interactive autoloaded Lisp function in `tabify.el
'.
(tabify START END &optional ARG)
Convert multiple spaces in region to tabs when possible.
A group of spaces is partially replaced by tabs
when this can be done without changing the column they end at.
If called interactively with prefix ARG, convert for the entire
buffer.
Called non-interactively, the region is specified by arguments
START and END, rather than by the position of point and mark.
The variable `tab-width' controls the spacing of tab stops.