The three levels of single-click, double-click, and triple-click select single caret position, current word, and whole line.

The way to get the behavior you want is to change teh definition of “word”, which you can do by setting Settings > Preferences > Delimiter > ☑ Add your characters… = :\

I know there have been PythonScript examples of highjacking double-click as well (using the scintilla notifications and callbacks), so if you didn’t want to change the definition of “word” you could solve it another way. NppExec can do a lot, but I don’t think it can handle the notification callbacks. NppExec might be able to create a script that goes from the current position, and searches both forward and backward until it hits a space in either direction – but I usually think of NppExec as more of a batch-scripting, for interacting the entire contents of the file with external programs, and think of character-based as something better suited for PythonScript or similar, so I’m not sure how I would go about solving that problem in NppExec.