Spell-check blacklisting word sequences

Question

I am a programmer and use Vim primarily for that, but I have found that enabling Vim's spell check is a somewhat useful feature to have just to help slightly incentivize cleaning up the parts of files that are written in English.

Now what I thought about just now is I think it would be cool if I could have this system mark certain arbitrary word sequences like "the the" as an error for me. Is this possible?

I know that I can use zw to mark a word as bad, but I want to do it for a sequence of words that I want to also treat as wrong (but which may be made up of words that are fine).

(use :set spell to enable spell check in Vim)

Answer 1

Alright, so I looked it up this way since I had Vim's source code lying around:

$ cd ~/Documents/vim
$ ack 'the the'

...

runtime/doc/spell.txt
399:It can also be used to recognize "the the" and highlight it.
1206:"the the" in the .dic file:
1208:   the the/! ~
1463:   REP the_the the ~

...

OK so Vim's documentation itself appears to explain the situation (though not to my own satisfaction):

WORD COMBINATIONS

It is possible to spell-check words that include a space. This is used to recognize words that are invalid when used by themselves, e.g. for "et al.". It can also be used to recognize "the the" and highlight it.

There's a bunch more documentation that goes in depth into how and what the spell system is capable of. It's mostly sort of boring.

I also discovered the best way to easily add a word sequence to the "bad spelling" list. Just use zw in visual mode after selecting the sequence you want to mark as bad (the mnemonic is w for "wrong"). It works great. What it does automatically is add the word sequence to the spellfile such as ~/.vim/spell/en.utf-8.add in a line ending with /! which means it's a "bad word".