Adapter to enable gradual migration from hashicorp/logutils to uber/zap

log.go

import (
	"io"
	"regexp"

	"go.uber.org/zap"
	"go.uber.org/zap/zapcore"
)

// Replaces hashicorp/logutils by directing Golang's log package to Zap but
// extracting the log level from any `[$severity]` pattern in the message.
// Intended to enable gradual migration to structured logging.
type ZapAdapter struct {
	logger       *zap.Logger
	defaultLevel zapcore.Level
	pattern      *regexp.Regexp
}

// Example: log.SetOutput(NewZapAdapter(zlog, zapcore.DebugLevel))
func NewZapAdapter(logger *zap.Logger, defaultLevel zapcore.Level) io.Writer {
	return &ZapAdapter{
		logger:       logger,
		defaultLevel: defaultLevel,
		pattern:      regexp.MustCompile(`\[(DEBUG|ERROR|INFO|WARN)\]`),
	}
}

func (a *ZapAdapter) Write(b []byte) (int, error) {
	lvl := a.defaultLevel
	submatches := a.pattern.FindSubmatch(b)
	if len(submatches) == 2 {
		plvl, err := zapcore.ParseLevel(string(submatches[1]))
		if err == nil {
			lvl = plvl
		}
	}
	a.logger.Log(lvl, string(b))
	return len(b), nil
}

Maybe use ^ to only look at the beginning of the string or do some prefix search:

regexp.MustCompile(`^\[(DEBUG|ERROR|INFO|WARN)\]`)

@pavelnikolov I had started with a prefix-only search then realised that hashicorp/logutils matches a severity pattern anywhere in the line, so I honoured that logic.
https://github.com/hashicorp/logutils/blob/127077f645bf3162cff69c6518f7da4f0b0409e2/level.go#L38-L46