That sounds like a Graphical User Interface event – do you have a particular editor in mind ?
Or do you simply want to repopulate the clipboard with the following line (if there is one, when and if a match is found) from the file ?
(in which case no editing app needs to be involved)
the text right underneath it
- The string in the same column ?
- At the same starting position defined as a character offset from the left ?
- The length of the text copied defined by word boundaries ?
As always, you would need to show us an example.