$re = '/\"transactionid\\\\\":\\\\\"(?<transactionid>[^\\\\]++)/';
$str = '[Source: {\\"commisionEvent\\":{\\"channel\\":\\"testString\\",\\"orderType\\":\\"testString\\",\\"eventSource\\":\\"testString\\",\\"eventCreationDate\\":\\"2018-02-10T00:30:21.298Z\\",\\"orderDate\\":\\"2018-02-10T00:30:21.298Z\\",\\"dealerCode\\":\\"testString\\",\\"ban\\":\\"123\\",\\"orderNo\\":\\"testString\\",\\"originalOrderNo\\":\\"testString\\",\\"customerName\\":\\"testString\\",\\"msisdn\\":\\"testString\\",\\"orderDetails\\":[{\\"sku\\":\\"testString\\",\\"imei\\":\\"testString\\",\\"msisdnLineLevel\\":\\"testString\\",\\"msrp\\":\\"1000.00\\",\\"customerPaidAmount\\":\\"1200.00\\",\\"sellingPrice\\":\\"1000.00\\",\\"jumpIndicator\\":\\"testString\\",\\"eipIndicator\\":\\"123\\",\\"eip1stPayment\\":\\"1100.00\\",\\"eipPlanId\\":\\"testString\\",\\"eipInitialAmount\\":\\"1000.00\\",\\"discount\\":\\"100.00\\",\\"transactionid\\":\\"testString\\",\\"shippedDate\\":\\"test\\",\\"priceoverrideamount\\":\\"1000.00\\",\\"priceOverrideCode\\":\\"testString\\",\\"overrideReason\\":\\"testString\\",\\"originalTransactionId\\":\\"testString\\",\\"lineType\\":\\"testString\\",\\"transactionType\\":\\"testString\\"}]}}; line: 1, column: 676]';
preg_match_all($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