Operates a bit like sed without having to rebuild a string from an array of lines.

filter.rb

LOG_PATTERN = /^(INFO|DEBUG)\t[-0-9]+ [:0-9]+\t/

def filter_and_format(log, level)
  log.each_line.with_object("") { |line, filtered|
    next unless line.start_with?(level)
    filtered << line.sub(LOG_PATTERN, '')
  }
end

Somewhat equivalent to:

sed -E "/^DEBUG/d; s/^(INFO|DEBUG) [-0-9]+ [:0-9]+ //"