But when I tried to print $machine_class then I got"9000" and "ia64" respectively for the above two examples.

Of course. Neither a space, nor a slash are word characters, so they aren't matched by \w

How to get the whole string?

/.*/. Add a /s modifier is the string can contain a newline.

I wanted to fetch whatever string appearing in the place of **** which keeps on changing for different instances. How to achieve that using REGEX?

/64 bit kernel 64 bit user (.*) AIX/s