How to make GREP select only numeric values?

I use the df command in a bash script:

df . -B MB | tail -1 | awk {'print $4'} | grep  .[0-9]*

This script returns:

99%

But I need only numbers (to make the next comparison). If I use the grep regex without the dot:

df . -B MB | tail -1 | awk {'print $4'} | grep  .[0-9]*

I receive nothing. How to fix?