Go: PascalSnake (e.g., Hello_World) un/marshaller

main.go

package main

import (
  "bytes"
  "regexp"
  "encoding/json"
)

type PascalSnakeMarshaller struct {
	Value interface{}
}

var keyMatchRegex = regexp.MustCompile(`\"(\w+)\":`)
var pascalSnakeRegex = regexp.MustCompile("(?P<l>[a-z])_(?P<r>[A-Z])")

func (s PascalSnakeMarshaller) MarshalJSON() ([]byte, error) {
	marshalled, err := json.Marshal(s.Value)
	if err != nil {
		return nil, err
	}

	converted := keyMatchRegex.ReplaceAllFunc(
		marshalled,
		func(match []byte) []byte {
			return bytes.ToLower(pascalSnakeRegex.ReplaceAll(
				match,
				[]byte("${l}_${r}"),
			))
		},
	)

	return converted, nil
}

func (s *PascalSnakeMarshaller) UnmarshalJSON(data []byte) error {
	m := make(map[string]interface{})
	err := json.Unmarshal(data, &m)
	if err != nil {
		return err
	}
	s.Value = m
	return nil
}