gtk-entry-completion-compute-prefix (completion key)

completion -- a gtk-entry-completion object
key -- a string with the text to complete for

The common prefix all rows starting with key or nil if no row matches key.

Computes the common prefix that is shared by all rows in completion that start with key. If no row matches key, nil will be returned. Note that a text column must have been set for this function to work, see the function gtk-entry-completion-text-column for details.