$re = '/MaximumBatchQuantity[^:]+:(?<MaximumBatchQuantity>[^,]+)/m';
$str = '{"bdy":{"msg":"HttpRequest","addInfo":[{"key":"Url","value":"https://isp0064x.st.ad.XXXXX.com/XXXXXOmniFulfillmentServerApp/api/Assignment/Auto/"},{"key":"Content","value":"{\\"Filters\\":[],\\"MaximumBatchQuantity\\":20,\\"AllowedSLAMilliSeconds\\":4500,\\"AssociateFirstName\\":\\"Aliona\\",\\"AssociateId\\":\\"2795969\\",\\"AssociateLastName\\":\\"Cieniawa\\",\\"Header\\":{\\"ApiVersion\\":null,\\"AppVersion\\":\\"18.3.0.15617\\",\\"JsonWebToken\\":null,\\"MessageId\\":\\"de959d4f-6a7d-4c0c-98d3-1143064b4300\\"},\\"IsOffline\\":false,\\"SLARequestKey\\":\\"AutoBatch\\",\\"StoreNumber\\":\\"0064\\"}"},{"key":"CorrelationId","value":""},{"key":"MessageId","value":"3a04038d-64e2-493c-b489-90a922de1980"}]},"hdr":{"level":"Verbose","timestamp":"2018-06-04T21:03:19.6347626Z","fxsrc":"LogRequestInfo","lineNum":710,"userId":"2795969","loc":"Store","locId":"0064","ip":"10.224.255.15","hostName":"K-W10ME-7463352","macaddress":"00-16-XX-16-A6-FA","eventid":0,"appVersion":"18.3.0.15617","appName":"OmniFulfillment","deviceModel":"XX500","osVersion":"10.0.14393.2007","firmwareVersion":"1049.7.18039.0","networkSignalStrength":"4","isConnected":"True"},"ver":"0.1"}';
preg_match_all($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