untabify
untabify is an interactive autoloaded Lisp function in `tabify.el
'.
(untabify START END &optional ARG)
Convert all tabs in region to multiple spaces, preserving columns.
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.