The text between point and the mark is known as the region. Various functions operate on text delimited by point and the mark, but only those functions specifically related to the region itself are described here.
The next two functions signal an error if the mark does not point
anywhere. If Transient Mark mode is enabled and
mark-even-if-inactive is nil, they also signal an error
if the mark is inactive.
This function returns the position of the beginning of the region (as an integer). This is the position of either point or the mark, whichever is smaller.
This function returns the position of the end of the region (as an integer). This is the position of either point or the mark, whichever is larger.
Instead of using region-beginning and region-end, a
command designed to operate on a region should normally use
interactive with the ‘r’ specification to find the
beginning and end of the region. This lets other Lisp programs
specify the bounds explicitly as arguments. See Code Characters for interactive.
This function returns t if Transient Mark mode is enabled, the
mark is active, and there is a valid region in the buffer. This
function is intended to be used by commands that operate on the
region, instead of on text near point, when the mark is active.
A region is valid if it has a non-zero size, or if the user option
use-empty-active-region is non-nil (by default, it is
nil). The function region-active-p is similar to
use-region-p, but considers all regions as valid. In most
cases, you should not use region-active-p, since if the region
is empty it is often more appropriate to operate on point.