Customising word separators in vi

vi treats dash - and space   as word separators for commands such as dw and cw.

Is there a way to add underscore _ as well?

I quite often want to change part of a variable name containing underscores, such as changing src_branch to dest_branch. I end up counting characters and using s (like 3sdest), but it would be much easier to use cw (like cwdest).

Is there a way to add underscore _ as well?

:set iskeyword-=_ 

What is, and is not a member character to keywords depends on the language. For help on iskeyword use :help iskeyword.

In case you're using vim, you can change that by setting the iskeyword option (:he iskeyword). If that is not an option, you can always use ct_ instead of counting.

One other good option in such cases is to use camelcasemotion plugin.

It adds new motions ,b, ,e, and ,w, which work analogously with b, e, and w, except that they recognize CamelCase and snake_case words. With it you can use

c,edest

and this will replace "src_branch" with "dest_branch" if your cursor was on first character of "src_branch".

You could type cf_dest_ and save the counting part.

Edit: or as suggested: ct_ changes text until right before the underline character. (I'm using the f motion more, so it came more naturally to me)

Or you could redefine 'iskeyword' (:help iskeyword for details).