use YAPE::Regex::Explain;
my $re = '[\w\d]+(^:[\w\d]+:$)\w+';
print YAPE::Regex::Explain->new($re)->explain();

__END__

The regular expression:

(?-imsx:[\w\d]+(^:[\w\d]+:$)\w+)

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):
----------------------------------------------------------------------
  [\w\d]+                  any character of: word characters (a-z, A-
                           Z, 0-9, _), digits (0-9) (1 or more times
                           (matching the most amount possible))
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    ^                        the beginning of the string
----------------------------------------------------------------------
    :                        ':'
----------------------------------------------------------------------
    [\w\d]+                  any character of: word characters (a-z,
                             A-Z, 0-9, _), digits (0-9) (1 or more
                             times (matching the most amount
                             possible))
----------------------------------------------------------------------
    :                        ':'
----------------------------------------------------------------------
    $                        before an optional \n, and the end of
                             the string
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  \w+                      word characters (a-z, A-Z, 0-9, _) (1 or
                           more times (matching the most amount
                           possible))
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------