gtk:editable-select-region (editable &key start end)

editable -- a gtk:editable widget
start -- an integer for the start of the region, the default value is 0
end -- an integer for the end of the region, the default value is -1

Selects a region of text. The characters that are selected are those characters at positions from start up to, but not including end. If end is negative, then the the characters selected are those characters from start to the end of the text. Note that positions are specified in characters, not bytes.