Yeah, fluentd... I had to look into its in_stdin plugin, and I can tell you one thing: it simply doesn't handle regexes properly. In fact, none of its input plugins can correctly handle the situation where a multiline regex doesn't match the whole line, unless they do what in_tail did (split the lines yourself and disallow the multiline regexes).

Edit: Sorry, I meant "Fluent bit". No idea how fluentd handle this scenario, but I was told it was too slow (being written in Ruby) so that's why the switch to fluentbit was made.