Install YAPE::Regex::Explain if you don't already have it, as it can break out a regex into a more verbose format that can help you spot things like that.

$ perl -MYAPE::Regex::Explain -le 'print YAPE::Regex::Explain->new(qr/
+([()\w\s-]+): \1/ )->explain'  
The regular expression:

(?-imsx:([()\w\s-]+): \1)

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:
----------------------------------------------------------------------
    [()\w\s-]+               any character of: '(', ')', word
                             characters (a-z, A-Z, 0-9, _),
                             whitespace (\n, \r, \t, \f, and " "), '-
                             ' (1 or more times (matching the most
                             amount possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  :                        ': '
----------------------------------------------------------------------
  \1                       what was matched by capture \1
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
[download]

there's also Graphviz::Regex to see a flow diagram of how a regex matches.