$re = '/endpointKeyHash\":\{\"string\":\"(?<endpointKeyHash>[^\=\"]*)/';
$str = '2641328 [EPS-log-dispatcher-11] INFO 1.24978294676695149906 - {"Log Header": "{"endpointKeyHash":{"string":"MAz7MadOhr02tPt5vtZsSEy9FWw="},"applicationToken":{"string":"24978294676695149906"},"headerVersion":{"int":1},"timestamp":{"long":1495594584490},"logSchemaVersion":{"int":2}}","Event":{"temperature":-1,"timeStamp":1495594583638}}';
preg_match_all($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