(?x:                     group, but do not capture (disregarding
                           whitespace and comments) (case-sensitive)
                           (with ^ and $ matching normally) (with .
                           not matching \n):
----------------------------------------------------------------------
    ^                        the beginning of the string
----------------------------------------------------------------------
    (?:                      group, but do not capture:
----------------------------------------------------------------------
      \d+                      digits (0-9) (1 or more times
                               (matching the most amount possible))
----------------------------------------------------------------------
      hmD                    any character of: 'h', 'm', 'D'
----------------------------------------------------------------------
    )                        end of grouping
----------------------------------------------------------------------
    $                        before an optional \n, and the end of
                             the string
----------------------------------------------------------------------
  )                        end of grouping

Thanks to YAPE::Regex::Explain