[Tutor] Regex not working as desired

[Albert-Jan Roskam]

On Feb 27, 2018 09:50, Alan Gauld via Tutor <tutor at python.org> wrote:
>
> On 27/02/18 05:13, Cameron Simpson wrote:
>
> > hard to debug when you do. That's not to say you shouldn't use them, but many
> > people use them for far too much.
>
>
> > Finally, you could also consider not using a regexp for this particular task.
> > Python's "int" class can be called with a string, and will raise an exception
>
> And, as another alternative, you can use all() with a
> generator expression:
>
> >>> all(c.isdigit() for c in '1234')
> True
> >>> all(c.isdigit() for c in '12c4')
> False

I never understood why this is syntactically correct. It's like two parentheses are missing.

This I understand:
all((c.isdigit() for c in '12c4'))
Or this:
all([c.isdigit() for c in '12c4'])
Or this:
all((True, False))

But the way you wrote it, the generator expression just "floats" in between the parentheses that are part of the all() function. Is this something special about all() and any()?