What is the pattern matched against for events CmdwinLeave and CmdwinEnter?

Question

More in general I'm always in doubt about what's the meaning of the {aupat} argument of autocmd for a given {event}.

From :help :autocmd I read (my emphasis)

Add {cmd} to the list of commands that Vim will execute automatically on {event} for a file matching {aupat} autocmd-patterns.

So from here it looks like {aupat} is always used maybe not to match a file name, but more in general to match a file. This kind of makes sense for events like BufWriteCmd, where {aupat} truly matching a file name, and FileType, where it matches the file type of the file.

But there are other events for which there's no such a thing as a file related to it. One example is ColorScheme, for which

The pattern is matched against the colorscheme name

reading directly from :help ColorScheme.

But not the documentation of all {event}s seem to explain what {aupat} means for them.

For instance, the word pattern doesn't even appear in :help CmdwinLeave and :help CmdwinEnter.

Hence the question: what is {aupat} matched against for {event}s CmdwinLeave and CmdwinEnter?

Answer 1

Those things are easy to test with :help <amatch>:

autocmd! CmdWinLeave * echomsg expand('<amatch>')

Where you use a wildcard to catch every possible case and echo <amatch> in the messages.

You get : when leaving q: and / when leaving q/, which lets you know what {aupat} to use in your autocommand:

autocmd! CmdWinLeave / <do something when leaving the search command-line window>

Answer 2

The documentation makes this clear I believe:

:h CmdwinLeave mentions this:

<afile> is set to a single character,
indicating the type of command-line.
|cmdwin-char|

(same for :h CmdwinEnter)

Note the |cmdwin-char|, which references another help topic, :h cmdwin-char, which finally answers your question:

The character used for the pattern indicates the type of command-line:
        :       normal Ex command
        >       debug mode command debug-mode
        /       forward search string
        ?       backward search string
        =       expression for "= expr-register
        @       string for input()
        -       text for :insert or :append