Obrigado pelas sugestões acima, mas depois de inúmeras tentativas, aqui estão as expressões que funcionaram.
[Definition]
ignoreregex =
failregex = \[client <HOST>\] File does not exist:.*(?i)MyAdmin.*
\[client <HOST>\] File does not exist:.*(?i)mysqlmanager.*
\[client <HOST>\] File does not exist:.*(?i)PMA.*
\[client <HOST>\] File does not exist:.*(?i)pma.*
\[client <HOST>\] File does not exist:.*(?i)php-my-admin.*
\[client <HOST>\] File does not exist:.*(?i)myadmin.*
\[client <HOST>\] File does not exist:.*(?i)administrator.*
\[client <HOST>\] File does not exist:.*(?i)xmlrpc.*
\[client <HOST>\] File does not exist:.*(?i)testproxy.*
\[client <HOST>\] File does not exist:.*(?i)phpMyAdmin.*
\[client <HOST>\] File does not exist:.*(?i)db.*
\[client <HOST>\] File does not exist:.*(?i)sql.*
Se alguém tiver plesk on centos, ele poderá usar essas regras para impedir ataques. Meu servidor continuava tendo o processo do apache sendo executado com 100% de uso da CPU.