$re = '/"GA_ClientID__c";[^"]*"([^"]*)"/m';
$str = '99524";s:9:"FirstName";s:2:"John";s:8:"LastName";s:8:"Doe";s:7:"Company";s:10:"Sample";s:5:"Email";s:20:"xxxxx@gmail.com";s:5:"Phone";s:10:"8888888888";s:7:"Country";s:13:"United States";s:5:"Title";s:8:"Creative";s:5:"State";s:2:"NC";s:13:"Last_Asset__c";s:40:"White Paper: Be a More Strategic Partner";s:16:"Last_Campaign__c";s:18:"70160000000q6TgAAI";s:16:"Referring_URL__c";s:8:"[direct]";s:19:"leadPriorityMarketo";s:2:"P2";s:18:"ProductInterest__c";s:9:"sample";s:14:"landingpageurl";s:359:"https://www.sample.com;mkt_tok=samplesamplesamplesample";s:14:"GA_ClientID__c";s:38:"75714ae471df63202106404675dasd800097erer1849995367";s:13:"Drupal_SID__c";s:36:"e1380c07-0258-47de-aaf8-82d4d8061e1a";s:4:"form";s:4:"1046";} ```
';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($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