Also assigning different sourcetype per county csv file is other option and you can handle it better for your future search queries.
In your input.conf file;
[monitor://C:\networkanalysisgermany.csv] disabled = false sourcetype = Germany [monitor://C:\networkanalysissingapore.csv] disabled = false sourcetype = Singapore
Hope it helps
I didn't test it but it should work if the length of the first part of the SOURCE string is always the same.
You can also apply a rex command after applying the split command, but it will do the same than the command proposed by sc0tt
I'm sure there is a more elegant solution, but would something like
| eval country = mvindex(split(substr(source,18),"."),0) work?
Edit: This should work as well
rex field=source "c:networkanalysis(?<country>\S+).csv"