use YAPE::Regex::Explain;
print YAPE::Regex::Explain->new(
qr{hd_defect-( ([^\.]+$) | (?: ([^\.]+).(.+$) ) )}sx
)->explain;
__END__
The regular expression:

(?sx-im:hd_defect-( ([^\.]+$) | (?: ([^\.]+).(.+$) ) ))

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?sx-im:                 group, but do not capture (with . matching
                         \n) (disregarding whitespace and comments)
                         (case-sensitive) (with ^ and $ matching
                         normally):
----------------------------------------------------------------------
  hd_defect-               'hd_defect-'
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    (                        group and capture to \2:
----------------------------------------------------------------------
      [^\.]+                   any character except: '\.' (1 or more
                               times (matching the most amount
                               possible))
----------------------------------------------------------------------
      $                        before an optional \n, and the end of
                               the string
----------------------------------------------------------------------
    )                        end of \2
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    (?:                      group, but do not capture:
----------------------------------------------------------------------
      (                        group and capture to \3:
----------------------------------------------------------------------
        [^\.]+                   any character except: '\.' (1 or
                                 more times (matching the most amount
                                 possible))
----------------------------------------------------------------------
      )                        end of \3
----------------------------------------------------------------------
      .                        any character
----------------------------------------------------------------------
      (                        group and capture to \4:
----------------------------------------------------------------------
        .+                       any character (1 or more times
                                 (matching the most amount possible))
----------------------------------------------------------------------
        $                        before an optional \n, and the end
                                 of the string
----------------------------------------------------------------------
      )                        end of \4
----------------------------------------------------------------------
    )                        end of grouping
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------