Allow leading underscore in sketch filenames

internal/rule/rulefunction/rulefunction.go

@@ -105,7 +105,7 @@ func brokenOutputList(list []string) string {
 
 // validProjectPathBaseName checks whether the provided library folder or sketch filename contains prohibited characters.
 func validProjectPathBaseName(name string) bool {
-	baseNameRegexp := regexp.MustCompile("^[a-zA-Z0-9][a-zA-Z0-9_.-]*$")
+	baseNameRegexp := regexp.MustCompile("^[a-zA-Z0-9_][a-zA-Z0-9_.-]*$")
 	return baseNameRegexp.MatchString(name)
 }
 

internal/rule/rulefunction/sketch_test.go

@@ -75,7 +75,7 @@ func TestSketchNameMismatch(t *testing.T) {
 func TestProhibitedCharactersInSketchFileName(t *testing.T) {
 	testTables := []sketchRuleFunctionTestTable{
 		{"Has prohibited characters", "ProhibitedCharactersInFileName", ruleresult.Fail, "^Prohibited CharactersInFileName.h$"},
-		{"No prohibited characters", "Valid", ruleresult.Pass, ""},
+		{"No prohibited characters", "AllowedCharactersInFilenames", ruleresult.Pass, ""},
 	}
 
 	checkSketchRuleFunction(ProhibitedCharactersInSketchFileName, testTables, t)

internal/rule/rulefunction/testdata/sketches/AllowedCharactersInFilenames/AllowedCharactersInFilenames.ino

@@ -0,0 +1,2 @@
+void setup() {}
+void loop() {}