When @ the linked post is written "lastSlashIndex" and "LastUndrIndex", are that funktions of painless? I did not find anything.
These are just arbitrary variable names that you can define in your script. Painless function is lastIndexOf. I'd recommend you first go through examples, other discuss threads and API reference. Experiment with them and get back if you still have questions.
As far as I know Regexes are disabled by default in Painless, but you can use other string-based functions.
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Oleg