Given your question here, and in other posts I suggest that you read up on regex in general.
e.g. http://www.regular-expressions.info
http://gskinner.com/RegExr/
In this case (one of) the answer(s) is;
rex field=source "/app/(?<item>[a-z]+)([^/]+/){2}.+(?<system>[^-]+)\.log$
Which is; find '/app/', then take any a-z characters and call them item . Then jump over any non-slash characters followed by a slash, twice. Then skip through any characters, until you find a set of non-dash characters followed by .log at the end of the string. Call these non-dash characters system .
/K
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