Create a one-line test log
$ cat /home/zabbix20/test_logs/test.log
error1

Create a new log[] item with invalid pattern "error.*\":
log[/home/zabbix20//test_logs/test.log,error.*\,,]

Agentd does not report anything to server as if there was no matching records in the log file. No errors about invalid regexp are logged.

Correct the invalid pattern to
log[/home/zabbix20//test_logs/test.log,error.*,,]

Now the record matches and is reported to server.