gtk:label-select-region (label start end)

label -- a gtk:label widget
start -- an integer for the start offset, in characters not bytes
end -- an integer for the end offset, in characters not bytes

Selects a range of characters in the label, if the label is selectable. See the selectable property. If the label is not selectable, this function has no effect. If start or end are -1, then the end of the label will be substituted.