Sorry, I didn't realize the html was a string constant.
I assumed you were reading from an actual html file,
which of course shouldn't have backslashes before the quotes.

But the extra backslash in your regex is definitely a problem.
Two backslashes in a row give a literal backslash, so you
have to remove one of them to give a proper \s.

Your problem may be that the period doesn't usually match
newlines. In Perl you make it do so with the /s option.
Perhaps there is something similar in boost.