Hi jithoosin, take a look at perlre. '?:' does not store the content in the memory. '?' is optional (either present or not) of the preceding content. To match normal '?', special character '?' has to be escaped by backslash '\?'. Here is explanation for that.

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
----------------------------------------------------------------------
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