Filtering regex from bash variable containing text

I have variable in bash:

$ users_="a1 .localhost a2 a3 .localhost a4"

I’d like to read into bash array all phrases startking with 'a' and ending with '.localhost', which in this case will be:

"a1 .localhost"
"a3 .localhost"

The regex should be I think a[0-9]+\s\.localhost or ^a[0-9]+\s\.localhost$ (I’m not sure). The a2 and a4 prases should be ommited.

So, how to achieve it?

What i was trying is:
$ $users_ | grep '^a[0-9]+\s\.localhost$'

But it throws me an error.

Thanks for replies.
Mike.