$re = '/(?<=id="_wpnonce" name="_wpnonce" value=").*?(?=")/';
$str = '<form method="post" enctype="multipart/form-data" class="wp-upload-form" action="http://localhost/wp-admin/update.php?action=upload-plugin">
<input type="hidden" id="_wpnonce" name="_wpnonce" value="a7a9167537"><input type="hidden" name="_wp_http_referer" value="/wp-admin/plugin-install.php?tab=upload"> <label class="screen-reader-text" for="pluginzip">Plugin zip file</label>
<input type="file" id="pluginzip" name="pluginzip">
<input type="submit" name="install-plugin-submit" id="install-plugin-submit" class="button" value="Install Now" disabled=""> </form>';
preg_match($re, $str, $matches, PREG_OFFSET_CAPTURE, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php