$re = '/^.*?(?P<body>{.*})/m';
$str = '{"channel":"crm_process_booking_log","log_type":"DEBUG","index_name":"crm_process_booking_log","index_type":"crm_process_booking_log","error_type":0,"message":{"data":{"STEP":"slot_details","slotDetail":{"stm_id":"684178718","time":"14:30:00","end_time":"15:30:00","locality_id":0,"booking_id":0,"freeze_count":0,"date":"2024-02-07","user_id":0,"collection_center_id":0,"b2b_zone_id":0,"zone_id":53,"sample_type_id":1,"isavailable":1,"isactive":1,"set_name":"set5","is_peak_hours":0,"source":"crm","order_group_id":0,"old_zone_id":0,"samplecollector_id":0,"old_set_name":"","phlebo_name":"","assign_time":"0000-00-00 00:00:00","freeze":0,"freeze_date":"0000-00-00 00:00:00","payment":0,"locusStatus":0,"lat":"","lng":"","dropTaskStatus":0,"autoAssignStatus":0,"pickup_type":"home","service_id":0,"consultation_id":0,"channel_type":0,"channel_user":0,"vendor_user_id":"","created_at":"2024-02-05 12:56:10","updated_at":"2024-02-05 12:56:10","reference_stm_id":0,"phlebo_id":0},"amzn_trace_id":"Root=1-65c33f1c-29fe535658001b076140677c"},"calling_trace":{"file":"\\/var\\/www\\/newcrm-t25\\/application\\/controllers\\/crmuser\\/crmorder.php","line":2112,"function":"addProcessBookingLog","class":"Enterprise_logger"}},"amzn_trace_id":"Root=1-65c33f1c-29fe535658001b076140677c","nginx_request_id":"c93176430784c9dd4e32e35541e6e31a","timestamp":"2024-02-07 13:58:12"}';
preg_match_all($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