Theses lasts days I checked how this mod works (with MC 1.12). And I figured some things:

• Filters are applied to all logs. This is great to understand but it have two huge drawbacks:
  • it can make false positives. This is especially true with short text filters or too generic filters
  • it can use a lot of CPU if there many filters
• Filters are applied to all outputs.
• Regex are compiled on the fly. So they are compiled again and again for each log message.
• The generic filter (for stdout) break the redirection to debug.log made by forge.
• Filter are not applied on the same way for Log4j logs and java.utils.logging.
• Texts filters are printed but not regex ones.
• There no example in config file

For the first point: what do you think of changing the config file to have per source filters ? (Only if the game can make blank filters in the config file)
For the second point: what about per output filters ? Or per configurable group of outputs (at lease one for all outputs and one for system console and server console only) ?
And what about per source and per output level filter ?

In all cases I will fix the others point first.