"Doesn't seem to work. #MO and # MO do not match."

That sounds like you forgot the 'i' modifier at the end of "/^#\s*[a-z]+/i".

"Tried the regex tester at http://www.internetofficer.com/seo-tool/regex-tester/ and it says "no match""

That site clearly states "We use Perl-style regular expressions." (not Perl regular expressions). It also seems to have limited functionality: it has a checkbox for the 'i' modifier and nothing for any of the other modifiers (see /PATTERN/msixpodualgc in "perlop: Regexp Quote-Like Operators").

Why not try it in a real perl script. As ++toolic provided you with the complete code, it's a simple copy-paste operation.