Why is my grep + regex not working?

If you want to use Perl regex syntax you need -P switch with grep. Check out previously asked guestion here Is grep syntax different from regex?

• You should quote your expression so the shell doesn't interpret it
• grep doesn't have the \d escape, you'll need to use [0-9] instead.
• + needs to be escaped without the -E switch.

This should work:

watch cat /proc/mdstat | grep 'finish=[0-9]\+\.[0-9]' | grep '[0-9]\+\.[0-9]'