Not good with grep but, first thing, are you using the correct switch for the job, and have you tested something really simple to see if that works? - I know from experience that these things can use some very peculiar variations of POSIX that don't follow conventional syntax.
Also, I think that the expression needs to be something more along the lines of:
...and I am not certain whether the escaped items in your code actually need to be escaped or not.
grep -E "\([:space:]|\'\)(.*)?\.domain\.com" index.html