diff --git a/internal/rule/rulefunction/rulefunction.go b/internal/rule/rulefunction/rulefunction.go
index 9659b78e..aebd745b 100644
--- a/internal/rule/rulefunction/rulefunction.go
+++ b/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)
 }
 
diff --git a/internal/rule/rulefunction/sketch_test.go b/internal/rule/rulefunction/sketch_test.go
index fa910d13..d873b0e0 100644
--- a/internal/rule/rulefunction/sketch_test.go
+++ b/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)
diff --git a/internal/rule/rulefunction/testdata/sketches/AllowedCharactersInFilenames/1LeadingNumber.ino b/internal/rule/rulefunction/testdata/sketches/AllowedCharactersInFilenames/1LeadingNumber.ino
new file mode 100644
index 00000000..e69de29b
diff --git a/internal/rule/rulefunction/testdata/sketches/AllowedCharactersInFilenames/AllowedCharactersInFilenames.ino b/internal/rule/rulefunction/testdata/sketches/AllowedCharactersInFilenames/AllowedCharactersInFilenames.ino
new file mode 100644
index 00000000..660bdbcc
--- /dev/null
+++ b/internal/rule/rulefunction/testdata/sketches/AllowedCharactersInFilenames/AllowedCharactersInFilenames.ino
@@ -0,0 +1,2 @@
+void setup() {}
+void loop() {}
diff --git a/internal/rule/rulefunction/testdata/sketches/AllowedCharactersInFilenames/Contains-Hyphen.ino b/internal/rule/rulefunction/testdata/sketches/AllowedCharactersInFilenames/Contains-Hyphen.ino
new file mode 100644
index 00000000..e69de29b
diff --git a/internal/rule/rulefunction/testdata/sketches/AllowedCharactersInFilenames/Contains.Dot.ino b/internal/rule/rulefunction/testdata/sketches/AllowedCharactersInFilenames/Contains.Dot.ino
new file mode 100644
index 00000000..e69de29b
diff --git a/internal/rule/rulefunction/testdata/sketches/AllowedCharactersInFilenames/ContainsNumber1.ino b/internal/rule/rulefunction/testdata/sketches/AllowedCharactersInFilenames/ContainsNumber1.ino
new file mode 100644
index 00000000..e69de29b
diff --git a/internal/rule/rulefunction/testdata/sketches/AllowedCharactersInFilenames/Contains_Underscore.ino b/internal/rule/rulefunction/testdata/sketches/AllowedCharactersInFilenames/Contains_Underscore.ino
new file mode 100644
index 00000000..e69de29b
diff --git a/internal/rule/rulefunction/testdata/sketches/AllowedCharactersInFilenames/_LeadingUnderscore.ino b/internal/rule/rulefunction/testdata/sketches/AllowedCharactersInFilenames/_LeadingUnderscore.ino
new file mode 100644
index 00000000..e69de29b