use re 'debug';
$m = ' more gibberish"h" \n URL="http://[10.0.0.3]?id=80943lkjh875kjrvf09u548gfpi"\n gibber\n';

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

if ($m =~ $u) {
    print "<$1><$2>\n";
}else{ print "no match u\n";}
__END__
Compiling REx `(:?URL="([^"]*)")?'
size 34 Got 276 bytes for offset annotations.
first at 1
   1: CURLYX[0] {0,1}(33)
   3:   OPEN1(5)
   5:     CURLY {0,1}(9)
   7:       EXACTF <:>(0)
   9:     EXACTF <URL=">(12)
  12:     OPEN2(14)
  14:       STAR(26)
  15:         ANYOF[\0-!#-\377{unicode_all}](0)
  26:     CLOSE2(28)
  28:     EXACTF <">(30)
  30:   CLOSE1(32)
  32:   WHILEM(0)
  33: NOTHING(34)
  34: END(0)
minlen 0 
Offsets: [34]
	18[1] 0[0] 1[1] 0[0] 3[1] 0[0] 2[1] 0[0] 4[5] 0[0] 0[0] 9[1] 0[0] 14[1] 10[4] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 15[1] 0[0] 16[1] 0[0] 17[1] 0[0] 18[0] 18[0] 19[0] 
Matching REx `(:?URL="([^"]*)")?' against ` more gibberish"h" \n URL="http://[10.0.0.3]?id=80943lkjh875...'
  Setting an EVAL scope, savestack=3
   0 <> < more gibber>    |  1:  CURLYX[0] {0,1}
   0 <> < more gibber>    | 32:    WHILEM
                              0 out of 0..1  cc=140fc18
  Setting an EVAL scope, savestack=9
   0 <> < more gibber>    |  3:      OPEN1
   0 <> < more gibber>    |  5:      CURLY {0,1}
                           EXACTF <:> can match 0 times out of 1...
  Setting an EVAL scope, savestack=9
                                failed...
     restoring \1..\2 to undef
                              failed, try continuation...
   0 <> < more gibber>    | 33:      NOTHING
   0 <> < more gibber>    | 34:      END
Match successful!
Freeing REx: `"(:?URL=\"([^\"]*)\")?"'

##</code><code>##

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