Allow leading underscore in sketch filenames (arduino#2105)

The Arduino Sketch Specification defines the allowed format of sketch folder names and sketch code filenames.

The origin of the specification is the text of the error message shown in Arduino IDE when the user attempts to save to
a name that contains disallowed characters:

https://github.com/arduino/Arduino/blob/89539b1131f8cde9f7a83225f21c811071af53a8/app/src/processing/app/SketchController.java#L847-L853

However, the implementation of the restriction in the IDE codebase has a bug that allows a leading underscore (because
the code uses _ as the replacement character).

After the restriction was implemented correctly in Arduino IDE 2.x, it was discovered that the loss of compatibility
with these non-compliant names was impactful.

One of the primary purposes for the specification and restrictions is to ensure sketches can be used in any tool. Since
the tools support this name format and there is no technical reason to disallow it, the best thing to do is change the
sketch specification to follow the historic tool behavior (even if that behavior was the result of a benign bug).

Leading underscores in sketch folder names and sketch code filenames are hereby permitted by the Arduino Sketch
Specification and `arduino-cli sketch new`.

Author: per1234

Diff for: commands/sketch/new.go

@@ -37,7 +37,7 @@ void loop() {
 
 // sketchNameMaxLength could be part of the regex, but it's intentionally left out for clearer error reporting
 var sketchNameMaxLength = 63
-var sketchNameValidationRegex = regexp.MustCompile(`^[0-9a-zA-Z][0-9a-zA-Z_\.-]*$`)
+var sketchNameValidationRegex = regexp.MustCompile(`^[0-9a-zA-Z_][0-9a-zA-Z_\.-]*$`)
 
 // NewSketch creates a new sketch via gRPC
 func NewSketch(ctx context.Context, req *rpc.NewSketchRequest) (*rpc.NewSketchResponse, error) {
@@ -80,7 +80,7 @@ func validateSketchName(name string) error {
 			sketchNameMaxLength))}
 	}
 	if !sketchNameValidationRegex.MatchString(name) {
-		return &arduino.CantCreateSketchError{Cause: errors.New(tr(`invalid sketch name "%[1]s": the first character must be alphanumeric, the following ones can also contain "_", "-", and ".".`,
+		return &arduino.CantCreateSketchError{Cause: errors.New(tr(`invalid sketch name "%[1]s": the first character must be alphanumeric or "_", the following ones can also contain "-" and ".".`,
 			name))}
 	}
 	return nil

Diff for: commands/sketch/new_test.go

@@ -13,7 +13,6 @@ func Test_SketchNameWrongPattern(t *testing.T) {
 	invalidNames := []string{
 		"&",
 		".hello",
-		"_hello",
 		"-hello",
 		"hello*",
 		"||||||||||||||",
@@ -25,7 +24,7 @@ func Test_SketchNameWrongPattern(t *testing.T) {
 			SketchDir:  t.TempDir(),
 		})
 
-		require.EqualError(t, err, fmt.Sprintf(`Can't create sketch: invalid sketch name "%s": the first character must be alphanumeric, the following ones can also contain "_", "-", and ".".`,
+		require.EqualError(t, err, fmt.Sprintf(`Can't create sketch: invalid sketch name "%s": the first character must be alphanumeric or "_", the following ones can also contain "-" and ".".`,
 			name))
 	}
 }
@@ -66,6 +65,7 @@ func Test_SketchNameOk(t *testing.T) {
 		"h..ello-world",
 		"h..ello-world.",
 		"hello_world__",
+		"_hello_world",
 		string(lengthLimitName),
 	}
 	for _, name := range validNames {

Diff for: docs/sketch-specification.md

@@ -5,10 +5,13 @@ The programs that run on Arduino boards are called "sketches". This term was inh
 
 ## Sketch folders and files
 
-The sketch root folder name and code file names must start with a basic letter (`A`-`Z` or `a`-`z`) or number (`0`-`9`),
-followed by basic letters, numbers, underscores (`_`), dots (`.`) and dashes (`-`). The maximum length is 63 characters.
+The sketch root folder name and code file names must start with a basic letter (`A`-`Z` or `a`-`z`), number (`0`-`9`)
+[<sup>1</sup>](#leading-number-note), or underscore (`_`) [<sup>2</sup>](#leading-underscore-note) followed by basic
+letters, numbers, underscores, dots (`.`) and dashes (`-`). The maximum length is 63 characters.
 
-Support for names starting with a number was added in Arduino IDE 1.8.4.
+<a id="leading-number-note"></a> <sup>1</sup> Supported from Arduino IDE 1.8.4. <br />
+<a id="leading-underscore-note"></a> <sup>2</sup> Supported in all versions except Arduino IDE 2.0.4/Arduino CLI
+0.30.0 - 0.30.1.
 
 ### Sketch root folder