$re = '/\"user\":\s+\{\s+\"id\":\s+(?<user_id>[^,]+)/m';
$str = '"aepassword": "kmdAkcu)n>Ec_.a(m5P7?8-n", "aeci": { "outgoing_server": "mailrv.aaa.com", "email_footer": "C:\\\\ProgramData\\\\bbb\\\\AutomationNote\\\\Email\\\\aa_Mail_Footer.png", "email_header": "C:\\\\ProgramData\\\\bbb\\\\AutomationNote\\\\Email\\\\aa_Mail_Header.png", "signature": "C:\\\\ProgramData\\\\bbb\\\\Automation\\\\Email\\\\bb_Email_Signature.txt", "requires_authentication": "false", "reply-to": "us@aaa.com", "primaryaddress": "ussdev@aaa.com", "host": "ussdev@bbb.com", "entity_alternate_names": "usdev@aaa.com", "outgoing_port": "2675", "entityid": "wmid-1607548215055521", "name": "bbb_MailBox", "entitytype": "Sub-System", "entitytype": "Workplace", "technology": "O736i85", "tenantid": 1000011, "cloudprovider": "", "satellite": "sat-16107579705752592", "resourceid": null, "UDetails": { "creds": { "email": "NA" }, "id": 14, "name": "N/A" }, "encryptionKey": "5inqhg7ckj7klk2w4osk0", "user": { "id": 5, "name": "CRI Admin", "employeecode": "125", "email": "admin@aaa.com" },';
preg_match_all($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