Marshal JSON in Golang using lower-camel-case object key conventions

main.go

package main

import (
	"encoding/json"
	"fmt"
	"regexp"
	"time"
	"unicode"
	"unicode/utf8"
)

// Regexp definitions
var keyMatchRegex = regexp.MustCompile(`\"(\w+)\":`)
var wordBarrierRegex = regexp.MustCompile(`(\w)([A-Z])`)

type conventionalMarshaller struct {
	Value interface{}
}

func (c conventionalMarshaller) MarshalJSON() ([]byte, error) {
	marshalled, err := json.Marshal(c.Value)

	converted := keyMatchRegex.ReplaceAllFunc(
		marshalled,
		func(match []byte) []byte {
			// Empty keys are valid JSON, only lowercase if we do not have an
			// empty key.
			if len(match) > 2 {
				// Decode first rune after the double quotes
				r, width := utf8.DecodeRune(match[1:])
				r = unicode.ToLower(r)
				utf8.EncodeRune(match[1:width+1], r)
			}
			return match
		},
	)

	return converted, err
}

type exampleModel struct {
	Title       string
	Description string
	CreatedAt   time.Time
	UpdatedAt   time.Time
	IsActive    bool
}

func main() {
	model := exampleModel{
		Title:       "Example Title",
		Description: "whatever",
		CreatedAt:   time.Now(),
		UpdatedAt:   time.Now(),
		IsActive:    true,
	}

	encoded, _ := json.MarshalIndent(conventionalMarshaller{model}, "", "  ")

	fmt.Println(string(encoded))
}

Adapted from here - https://gist.github.com/Rican7/39a3dc10c1499384ca91 that gist shows snake case conversion, this handles lowerCamelCase.