The regular expression:

(?-imsx:(d{4,5}).htm)

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):
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    d{4,5}                   'd' (between 4 and 5 times (matching the
                             most amount possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  .                        any character except \n
----------------------------------------------------------------------
  htm                      'htm'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------