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

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

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