use YAPE::Regex::Explain;
$u= qr/(:?URL="([^"]*)")?/is; # optional clause (:?xxx)?

print YAPE::Regex::Explain->new($u)->explain;
__END__

The regular expression:

(?is-mx:(:?URL="([^"]*)")?)

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?is-mx:                 group, but do not capture (case-insensitive)
                         (with . matching \n) (with ^ and $ matching
                         normally) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  (                        group and capture to \1 (optional
                           (matching the most amount possible)):
----------------------------------------------------------------------
    :?                       ':' (optional (matching the most amount
                             possible))
----------------------------------------------------------------------
    URL="                    'URL="'
----------------------------------------------------------------------
    (                        group and capture to \2:
----------------------------------------------------------------------
      [^"]*                    any character except: '"' (0 or more
                               times (matching the most amount
                               possible))
----------------------------------------------------------------------
    )                        end of \2
----------------------------------------------------------------------
    "                        '"'
----------------------------------------------------------------------
  )?                       end of \1 (NOTE: because you're using a
                           quantifier on this capture, only the LAST
                           repetition of the captured pattern will be
                           stored in \1)
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------