Add string replacer interface and env key replacer option

README.md

@@ -263,6 +263,9 @@ keys to an extent. This is useful if you want to use `-` or something in your
 `Get()` calls, but want your environmental variables to use `_` delimiters. An
 example of using it can be found in `viper_test.go`.
 
+Alternatively, you can use `EnvKeyReplacer` with `NewWithOptions` factory function.
+Unlike `SetEnvKeyReplacer`, it accepts a `StringReplacer` interface allowing you to write custom string replacing logic.
+
 By default empty environment variables are considered unset and will fall back to
 the next configuration source. To treat empty environment variables as set, use
 the `AllowEmptyEnv` method.

viper.go

@@ -197,7 +197,7 @@ type Viper struct {
 	envPrefix         string
 
 	automaticEnvApplied bool
-	envKeyReplacer      *strings.Replacer
+	envKeyReplacer      StringReplacer
 	allowEmptyEnv       bool
 
 	config         map[string]interface{}
@@ -257,6 +257,19 @@ func KeyDelimiter(d string) Option {
 	})
 }
 
+// StringReplacer applies a set of replacements to a string.
+type StringReplacer interface {
+	// Replace returns a copy of s with all replacements performed.
+	Replace(s string) string
+}
+
+// EnvKeyReplacer sets a replacer used for mapping environment variables to internal keys.
+func EnvKeyReplacer(r StringReplacer) Option {
+	return optionFunc(func(v *Viper) {
+		v.envKeyReplacer = r
+	})
+}
+
 // NewWithOptions creates a new Viper instance.
 func NewWithOptions(opts ...Option) *Viper {
 	v := New()

viper_test.go

@@ -549,6 +549,15 @@ func TestSetEnvKeyReplacer(t *testing.T) {
 	assert.Equal(t, "30s", Get("refresh-interval"))
 }
 
+func TestEnvKeyReplacer(t *testing.T) {
+	v := NewWithOptions(EnvKeyReplacer(strings.NewReplacer("-", "_")))
+
+	v.AutomaticEnv()
+	_ = os.Setenv("REFRESH_INTERVAL", "30s")
+
+	assert.Equal(t, "30s", v.Get("refresh-interval"))
+}
+
 func TestAllKeys(t *testing.T) {
 	initConfigs()