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
----------------------------------------------------------------------
[download]

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

\.        # the character '.'
swf       # the string, 'swf'
(?: ... ) # don't save this expression
\?        # the character '?'
.*        # zero or more characters (not a newline)
?         # zero or one occurances (of the grouping in parens)
$         # end of the line
/oi       # don't re-evaluate, case insensitive.
[download]

So, as best I can tell, you're not only grabbing URLs that end in SWF, you're also grabbing URLs that are passing a query string (ie, URL?QUERY_STRING format)

It depends on which "?" you mean.

The typical use of "?" in a regex is "zero or one" of the preceding thing.Of course, another way to say "zero or one", is "optional".  So, for example, in "(?:\?.*)?", the final "?" means that "(?:\?.*)" is optional.

The second "?" (in "\?") stands for a single "?" (since it's escaped with "\").

However, the first "?":  :"(?:...)" has a different meaning, which is that it turns off the "capture" that would usually happen with "(...)".

You should read more about perl regular expressions to get the full scoop.

Others have answered your question. Basically you have three different types of question marks: ?:, \? and ? - non capturing matching, escaping, and zero or more matches. But looking at that regex, I wonder if it's all just overkill. It looks like your matching a flash url that may have paramaters. I wonder if this simpler regex will do the job as well:

m/\.swf.*$/oi
[download]

Maybe, maybe not. I think it's overkill, but your proposal is too simple. It might cause false matches if there were a 'www.swf.com' (or even 'www.swfa.com')

I'd probably use something like:

m/[.]swf(?:$|[?])/i

if we're not capturing, there's no reason to need '.*' to get the rest of the line, so a simple alternate (end of line, or a question mark), will work just fine.

It's almost as few characters to use the following, and there would hopefully be less confusion about what it's doing for novices:

m/\.swf$|\.swf\?/i