use YAPE::Regex::Explain;

$REx = 'm/\.swf(?:\?.*)?$/oi';

my $exp = YAPE::Regex::Explain->new($REx)->explain;

print $exp;

output:
-------

The regular expression:

(?-imsx:m/\.swf(?:\?.*)?$/oi)

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):
----------------------------------------------------------------------
  m/                       'm/'
----------------------------------------------------------------------
  \.                       '.'
----------------------------------------------------------------------
  swf                      'swf'
----------------------------------------------------------------------
  (?:                      group, but do not capture (optional
                           (matching the most amount possible)):
----------------------------------------------------------------------
    \?                       '?'
----------------------------------------------------------------------
    .*                       any character except \n (0 or more times
                             (matching the most amount possible))
----------------------------------------------------------------------
  )?                       end of grouping
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
  /oi                      '/oi'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------