Condition inside regex pattern

PHP preg_replace_callback(), (?R), (?1), (?2)..., . script :

<?php // test.php 20110312_2200

function clean_non_code(&$text) {
    $re = '%
    # Match and capture either CODE into $1 or non-CODE into $2.
      (                      # $1: CODE section (never empty).
        <code[^>]*>          # CODE opening tag
        (?R)+                # CODE contents w/nested CODE tags.
        </code\s*>           # CODE closing tag
      )                      # End $1: CODE section.
    |                        # Or...
      (                      # $2: Non-CODE section (may be empty).
        [^<]*+               # Zero or more non-< {normal*}
        (?:                  # Begin {(special normal*)*}
          (?!</?code\b)      # If not a code open or close tag,
          <                  # match non-code < {special}
          [^<]*+             # More {normal*}
        )*+                  # End {(special normal*)*}
      )                      # End $2: Non-CODE section
    %ix';

    $text = preg_replace_callback($re, '_my_callback', $text);
    if ($text === null) exit('PREG Error!\nTarget string too big.');
    return $text;
}

// The callback function is called once for each
// match found and is passed one parameter: $matches.
function _my_callback($matches)
{ // Either $1 or $2 matched, but never both.
    if ($matches[1]) {
        return $matches[1];
    }
    // Collapse multiple tabs and spaces into a single space.
    $matches[2] = preg_replace('/[ \t][ \t]++/S', ' ', $matches[2]);
    // Trim each line
    $matches[2] = preg_replace('/^ /m', '', $matches[2]);
    $matches[2] = preg_replace('/ $/m', '', $matches[2]);
    return $matches[2];
}

// Create some test data.
$data = "This  is some  text
  This is also   some text

<code>
User::setup(array(
    'key1'      => 'value1',
    'key2'      => 'value1',
    'key42'     => '<code>
        Pay no attention to this. It has been proven over and
        over again that it is <code>   unpossible   </code>
        to parse nested stuff with regex!           </code>'
));
</code>";

// Demonstrate that it works on one small test string.
echo("BEFORE:\n". $data ."\n\n");
echo("AFTER:\n". clean_non_code($data) ."\n\nTesting...");

// Build a large test string.
$bigdata = '';
for ($i =   0; $i < 30000; ++$i) $bigdata .= $data;
$size = strlen($bigdata);

// Measure how long it takes to process it.
$time = microtime(true);
$bigdata = clean_non_code($bigdata);
$time = microtime(true) - $time;

// Print benchmark results
printf("Done.\nTest string size: %d bytes. Time: %.3f sec. Speed: %.0f KB/s.\n",
    $size, $time, ($size / $time)/1024.);
?>

script : WinXP32 PHP 5.2.14 (cli)

'Test string size: 10410000 bytes. Time: 1.219 sec. Speed: 8337 KB/s.'

, CODE, <> (, ), . , ( , CODE .)

p.s. SO. <!-- language: lang-none --> .