In windows event log monitoring, the parameter <source>:

eventlog[name,<regexp>,<severity>,<source>,<eventid>,<maxlines>,<mode>]

Matches case insensitive. Seems that this is done on purpose:
https://git.zabbix.com/projects/ZBX/repos/zabbix/browse/src/zabbix_agent/eventlog.c#1040-1047

Example of this would look like this:

eventlog[System,,Error,"^MYTEST$",,,skip]

and all messages like these are matched:

MYTEST Error 999 This is a test 
MYTest Error 999 This is a test 
MYTesT Error 999 This is a test 

Please include this detail in the documentation.