ToSnake function for golang, convention-compliant

snake.go

package main

import (
	"unicode"
)

// ToSnake convert the given string to snake case following the Golang format:
// acronyms are converted to lower-case and preceded by an underscore.
func ToSnake(in string) string {
	runes := []rune(in)
	length := len(runes)

	var out []rune
	for i := 0; i < length; i++ {
		if i > 0 && unicode.IsUpper(runes[i]) && ((i+1 < length && unicode.IsLower(runes[i+1])) || unicode.IsLower(runes[i-1])) {
			out = append(out, '_')
		}
		out = append(out, unicode.ToLower(runes[i]))
	}

	return string(out)
}

snake_test.go

package main

import (
	"strings"
	"testing"
)

type SnakeTest struct {
	input  string
	output string
}

var tests = []SnakeTest{
	{"a", "a"},
	{"snake", "snake"},
	{"A", "a"},
	{"ID", "id"},
	{"MOTD", "motd"},
	{"Snake", "snake"},
	{"SnakeTest", "snake_test"},
	{"SnakeID", "snake_id"},
	{"SnakeIDGoogle", "snake_id_google"},
	{"LinuxMOTD", "linux_motd"},
	{"OMGWTFBBQ", "omgwtfbbq"},
	{"omg_wtf_bbq", "omg_wtf_bbq"},
}

func TestToSnake(t *testing.T) {
	for _, test := range tests {
		if ToSnake(test.input) != test.output {
			t.Errorf(`ToSnake("%s"), wanted "%s", got \%s"`, test.input, test.output, ToSnake(test.input))
		}
	}
}

var benchmarks = []string{
	"a",
	"snake",
	"A",
	"Snake",
	"SnakeTest",
	"SnakeID",
	"SnakeIDGoogle",
	"LinuxMOTD",
	"OMGWTFBBQ",
	"omg_wtf_bbq",
}

func BenchmarkToSnake(b *testing.B) {
	for i := 0; i < b.N; i++ {
		for _, input := range benchmarks {
			ToSnake(input)
		}
	}
}

func BenchmarkToLower(b *testing.B) {
	for i := 0; i < b.N; i++ {
		for _, input := range benchmarks {
			strings.ToLower(input)
		}
	}
}

Slightly modified version to include snake casing when strings contain numbers e.g. http2xx becomes http_2xx.

// ToSnakeCase convert the given string to snake case following the Golang format:
// acronyms are converted to lower-case and preceded by an underscore.
func ToSnakeCase(in string) string {
	runes := []rune(in)

	var out []rune
	for i := 0; i < len(runes); i++ {
		if i > 0 && (unicode.IsUpper(runes[i]) || unicode.IsNumber(runes[i])) && ((i+1 < len(runes) && unicode.IsLower(runes[i+1])) || unicode.IsLower(runes[i-1])) {
			out = append(out, '_')
		}
		out = append(out, unicode.ToLower(runes[i]))
	}

	return string(out)
}