main.go

import (
 "os"
 "regexp"
 log "github.com/sirupsen/logrus"
)

type LogWriter struct{}

var levelRegex *regexp.Regexp

const (
 LevelError   = "error"
 LevelWarning = "warning"
 LevelFatal   = "fatal"
 LevelPanic   = "panic"
)

func init() {
 var err error
 levelRegex, err = regexp.Compile("level=([a-z]+)")
 if err != nil {
  log.WithError(err).Fatal("Cannot setup log level")
 }
}

func (w *LogWriter) detectLogLevel(p []byte) (level string) {
 matches := levelRegex.FindStringSubmatch(string(p))
 if len(matches) > 1 {
  level = matches[1]
 }
 return
}

func (w *LogWriter) Write(p []byte) (n int, err error) {
 level := w.detectLogLevel(p)
  if level == LevelError || level == LevelWarning || level == LevelFatal || level == LevelPanic {
  return os.Stderr.Write(p)
 }
 return os.Stdout.Write(p)
}

func main() {
 log.SetOutput(&LogWriter{})
}