grep --perl-regexp and empty match

[Jori Mantysalo]

I was trying to get "one" and "two" from string "<B>one</B> and
<B>two</B>". Of course this doesn't work:
 grep -o -E '<B>.*</B>'
because it match hole string. So, doing it with perl-style regexp:
 grep -o -P '<B>.*?</B>'
well, that works. However, if I dont want tags, just whats inside,
 grep -o -P '(?<=<B>)(.+?)(?=</B>)'
OK, but a problem:
 echo '<B></B> and <B>two</B>' | grep -o -P '(?<=<B>)(.+?)(?=</B>)'
Well, maybe with star:
 echo '<B></B> and <B>two</B>' | grep -o -P '(?<=<B>)(.*?)(?=</B>)'
but now I got no output at all. So, is there any way to get an empty
match from grep?

-- 
Jori Mäntysalo