$re = '/(?<variable><variable.+?source=[\\\\]*"(?<source>.*?)[\\\\]*".*?>.*?<\/variable>)/';
$str = 'customtracking=\\"[{"name":"utm_source","value":"Loopify"},{"name":"utm_campaign","value":""},{"name":"utm_medium","value":"Page"},{"name":"utm_content","value":"<variable contenteditable=\\\\"false\\\\" name=\\\\"link type\\\\" source=\\\\"campaign.linkType\\\\" placeholder=\\\\"\\\\" date-locale=\\\\"\\\\" date-format=\\\\"\\\\">link type</variable>"}]\\" class=\\"loopify-link\\">aliquam</a>, lobortis ante eu, consectetur nulla. Sed vitae posuere purus. Vivamus pretium et ipsum vel pretium. Nunc elementum tempus magna id iaculis.","button1":"Google","button2":"Read more"},"link1":{"type":"website","url":"https://www.google.com","uuid":"1a61717c-a686-4683-450c-5fa775b74b6f","target":"_blank","title":"","customTracking":[{"name":"utm_source","value":"Loopify"},{"name":"utm_campaign","value":"<variable contenteditable=\\"false\\" name=\\"campaign name\\" source=\\"campaign.campaignName\\" placeholder=\\"\\" date-locale=\\"\\" date-format=\\"\\">campaign name</variable>"},{"name":"utm_medium","value":"Page"},{"name":"utm_content","value":"<variable contenteditable=\\"false\\" name=\\"link type\\" source=\\"campaign.linkType\\" placeholder=\\"\\" date-locale=\\"\\" date-format=\\"\\">link type</variable>"}]}';
preg_match_all($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