use YAPE::Regex::Explain;
print YAPE::Regex::Explain->new("\\B")->explain;
__END__
The regular expression:

(?-imsx:\B)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  \B                       the boundary between two word chars (\w)
                           or two non-word chars (\W)
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------