fix #39 and refs #1. add rules to convert strings with numbers.

convert.go

@@ -81,6 +81,9 @@ func ToCamelCase(str string) string {
 //     "GO_PATH"    => "go_path"
 //     "GO PATH"    => "go_path"      // space is converted to underscore.
 //     "GO-PATH"    => "go_path"      // hyphen is converted to underscore.
+//     "HTTP2XX"    => "http_2xx"     // insert an underscore before a number and after an alphabet.
+//     "http2xx"    => "http_2xx"
+//     "HTTP20xOK"  => "http_20x_ok"
 func ToSnakeCase(str string) string {
 	if len(str) == 0 {
 		return ""
@@ -102,7 +105,7 @@ func ToSnakeCase(str string) string {
 			buf.WriteByte(byte(str[0]))
 
 		case unicode.IsUpper(r0):
-			if prev != '_' {
+			if prev != '_' && !unicode.IsNumber(prev) {
 				buf.WriteRune('_')
 			}
 
@@ -140,6 +143,12 @@ func ToSnakeCase(str string) string {
 						r0 = '_'
 
 						buf.WriteRune(unicode.ToLower(r1))
+					} else if unicode.IsNumber(r0) {
+						// treat a number as an upper case rune
+						// so that both `http2xx` and `HTTP2XX` can be converted to `http_2xx`.
+						buf.WriteRune(unicode.ToLower(r1))
+						buf.WriteRune('_')
+						buf.WriteRune(r0)
 					} else {
 						buf.WriteRune('_')
 						buf.WriteRune(unicode.ToLower(r1))
@@ -154,9 +163,15 @@ func ToSnakeCase(str string) string {
 
 			if len(str) == 0 || r0 == '_' {
 				buf.WriteRune(unicode.ToLower(r0))
-				break
 			}
 
+		case unicode.IsNumber(r0):
+			if prev != '_' && !unicode.IsNumber(prev) {
+				buf.WriteRune('_')
+			}
+
+			buf.WriteRune(r0)
+
 		default:
 			if r0 == ' ' || r0 == '-' {
 				r0 = '_'

convert_test.go

@@ -22,6 +22,11 @@ func TestToSnakeCase(t *testing.T) {
 		"HELLO____WORLD":     "hello____world",
 		"TW":                 "tw",
 		"_C":                 "_c",
+		"http2xx":            "http_2xx",
+		"HTTP2XX":            "http_2xx",
+		"HTTP20xOK":          "http_20x_ok",
+		"HTTP20XStatus":      "http_20x_status",
+		"HTTP-20xStatus":     "http_20x_status",
 
 		"  sentence case  ":                                    "__sentence_case__",
 		" Mixed-hyphen case _and SENTENCE_case and UPPER-case": "_mixed_hyphen_case__and_sentence_case_and_upper_case",

go.mod

@@ -0,0 +1 @@
+module github.com/huandu/xstrings