diff --git a/docs/rules/0191/filenames.md b/docs/rules/0191/filenames.md
index 48f9cfba2..601a785a4 100644
--- a/docs/rules/0191/filenames.md
+++ b/docs/rules/0191/filenames.md
@@ -21,6 +21,7 @@ paths), it is important not to have odd paths.
 This rule currently complains if the filename:
 
 - ...is set to the proto version.
+- ...contains invalid cahracters.
 
 ## Examples
 
@@ -28,6 +29,11 @@ This rule currently complains if the filename:
 
 - `v1.proto`
 - `v1beta1.proto`
+- `library.service.proto`
+- `library#.proto`
+- `library$.proto`
+- `library service.proto`
+- `library_Service.proto`
 
 **Correct** filenames for this rule:
 
diff --git a/rules/aip0191/aip0191.go b/rules/aip0191/aip0191.go
index 42cd4db89..f89f7d099 100644
--- a/rules/aip0191/aip0191.go
+++ b/rules/aip0191/aip0191.go
@@ -44,4 +44,5 @@ func hasPackage(f *desc.FileDescriptor) bool {
 	return f.GetPackage() != ""
 }
 
-var versionRegexp = regexp.MustCompile("^v[0-9]+(p[0-9]+)?((alpha|beta)[0-9]*)?$")
+var versionRegexp = regexp.MustCompile(`^v[0-9]+(p[0-9]+)?((alpha|beta)[0-9]*)?$`)
+var validCharacterRegexp = regexp.MustCompile(`^[a-z0-9\\_\\/]*$`)
diff --git a/rules/aip0191/filenames.go b/rules/aip0191/filenames.go
index 0c801af8b..195ef6bfc 100644
--- a/rules/aip0191/filenames.go
+++ b/rules/aip0191/filenames.go
@@ -34,6 +34,13 @@ var filename = &lint.FileRule{
 				Location:   locations.FilePackage(f),
 			}}
 		}
+		if !validCharacterRegexp.MatchString(fn) {
+			return []lint.Problem{{
+				Message:    "The filename has invalid characters.",
+				Descriptor: f,
+				Location:   locations.FilePackage(f),
+			}}
+		}
 		return nil
 	},
 }
diff --git a/rules/aip0191/filenames_test.go b/rules/aip0191/filenames_test.go
index a39d515cc..9849b0f15 100644
--- a/rules/aip0191/filenames_test.go
+++ b/rules/aip0191/filenames_test.go
@@ -29,6 +29,12 @@ func TestFilename(t *testing.T) {
 	}{
 		{"Valid", "library.proto", testutils.Problems{}},
 		{"ValidDirectory", "google/library.proto", testutils.Problems{}},
+		{"ValidFileNameWithSnakeCase", "library_test.proto", testutils.Problems{}},
+		{"InvalidFileNameNotSnakeCase", "library.test.proto", testutils.Problems{{Message: "invalid characters"}}},
+		{"InvalidCharacterUpperCase", "library_Test.proto", testutils.Problems{{Message: "invalid characters"}}},
+		{"InvalidCharacterDollar", "library_$test.proto", testutils.Problems{{Message: "invalid characters"}}},
+		{"InvalidCharacterSpace", "library test.proto", testutils.Problems{{Message: "invalid characters"}}},
+		{"InvalidCharacterHash", "library_#test.proto", testutils.Problems{{Message: "invalid characters"}}},
 		{"InvalidStable", "v1.proto", testutils.Problems{{Message: "proto version"}}},
 		{"InvalidBigStable", "v20.proto", testutils.Problems{{Message: "proto version"}}},
 		{"InvalidStableDirectory", "google/library/v1.proto", testutils.Problems{{Message: "proto version"}}},