Comments - Do not understand why MariaDB rejected REGEXP with 42000 You have an error in your SQL syntax
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I still do not know how to perform the trace. For example I tried the steps described on https://dbtut.com/index.php/2018/11/15/how-to-get-the-data-packets-between-mysql-client-and-server/
$ sudo tcpdump -nnei any port 8001 -w tmp.pcap [sudo] password for jcobban: tcpdump: listening on any, link-type LINUX_SLL (Linux cooked v1), capture size 262144 bytes
At this point I invoked the script http://localhost/Ontario/WmbResponse.php?district=Oxford+%26+Middlesex&area=Nissouri+Twp.E%26W which duplicated the error.
^C0 packets captured 0 packets received by filter 0 packets dropped by kernel
The error is on requests made from PHP to the MariaDB server, not on requests from the mysql client.