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):
----------------------------------------------------------------------
  (?<=                     look behind to see if there is:
----------------------------------------------------------------------
    B                        'B'
----------------------------------------------------------------------
  )                        end of look-behind
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    -+                       '-' (1 or more times (matching the most
                             amount possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------