diff --git a/gopmod/classfile.go b/gopmod/classfile.go
index 49a6260..1721d5d 100644
--- a/gopmod/classfile.go
+++ b/gopmod/classfile.go
@@ -18,7 +18,6 @@ package gopmod
 
 import (
 	"errors"
-	"path"
 	"syscall"
 
 	"github.com/goplus/mod/modcache"
@@ -47,7 +46,7 @@ var (
 // -----------------------------------------------------------------------------
 
 func (p *Module) ClassKind(fname string) (isProj, ok bool) {
-	ext := path.Ext(fname)
+	ext := modfile.ClassExt(fname)
 	if c, ok := p.projects[ext]; ok {
 		for _, w := range c.Works {
 			if w.Ext == ext {
diff --git a/modfile/ext.go b/modfile/ext.go
new file mode 100644
index 0000000..dccd2db
--- /dev/null
+++ b/modfile/ext.go
@@ -0,0 +1,75 @@
+/*
+ * Copyright (c) 2021 The GoPlus Authors (goplus.org). All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package modfile
+
+import (
+	"fmt"
+	"path"
+	"strings"
+
+	"github.com/qiniu/x/errors"
+)
+
+// can be "_[class].gox" or ".[class]"
+func isExt(s string) bool {
+	return len(s) > 1 && (s[0] == '_' || s[0] == '.')
+}
+
+func parseExt(s *string) (t string, err error) {
+	t, err = parseString(s)
+	if err != nil {
+		goto failed
+	}
+	if isExt(t) {
+		return
+	}
+	err = errors.New("invalid ext format")
+failed:
+	return "", &InvalidExtError{
+		Ext: *s,
+		Err: err,
+	}
+}
+
+type InvalidExtError struct {
+	Ext string
+	Err error
+}
+
+func (e *InvalidExtError) Error() string {
+	return fmt.Sprintf("ext %s invalid: %s", e.Ext, e.Err)
+}
+
+func (e *InvalidExtError) Unwrap() error { return e.Err }
+
+// SplitFname splits fname into (hasGoxExt, className, classExt).
+func SplitFname(fname string) (hasGoxExt bool, className, classExt string) {
+	classExt = path.Ext(fname)
+	className = fname[:len(fname)-len(classExt)]
+	if hasGoxExt = (classExt == ".gox"); hasGoxExt {
+		if n := strings.LastIndexByte(className, '_'); n > 0 {
+			className, classExt = fname[:n], fname[n:]
+		}
+	}
+	return
+}
+
+// ClassExt returns classExt of specified fname.
+func ClassExt(fname string) string {
+	_, _, ext := SplitFname(fname)
+	return ext
+}
diff --git a/modfile/ext_test.go b/modfile/ext_test.go
new file mode 100644
index 0000000..5b5fb53
--- /dev/null
+++ b/modfile/ext_test.go
@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2021 The GoPlus Authors (goplus.org). All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package modfile
+
+import (
+	"testing"
+)
+
+func TestSplitFname(t *testing.T) {
+	type testCase struct {
+		fname string
+		ext   string
+	}
+	cases := []testCase{
+		{"foo.spx", ".spx"},
+		{"foo_yap.gox", "_yap.gox"},
+		{"foo.gox", ".gox"},
+	}
+	for _, c := range cases {
+		if ext := ClassExt(c.fname); ext != c.ext {
+			t.Fatalf("ClassExt(%s): expect %s, got %s\n", c.fname, c.ext, ext)
+		}
+	}
+}
diff --git a/modfile/rule.go b/modfile/rule.go
index 57c81ea..0e103a3 100644
--- a/modfile/rule.go
+++ b/modfile/rule.go
@@ -340,38 +340,6 @@ func isPkgPath(s string) bool {
 	return s != "" && (s[0] != '.' && s[0] != '_')
 }
 
-// can be "_[class].gox" or ".[class]"
-func isExt(s string) bool {
-	return len(s) > 1 && (s[0] == '_' || s[0] == '.')
-}
-
-func parseExt(s *string) (t string, err error) {
-	t, err = parseString(s)
-	if err != nil {
-		goto failed
-	}
-	if isExt(t) {
-		return
-	}
-	err = errors.New("invalid ext format")
-failed:
-	return "", &InvalidExtError{
-		Ext: *s,
-		Err: err,
-	}
-}
-
-type InvalidExtError struct {
-	Ext string
-	Err error
-}
-
-func (e *InvalidExtError) Error() string {
-	return fmt.Sprintf("ext %s invalid: %s", e.Ext, e.Err)
-}
-
-func (e *InvalidExtError) Unwrap() error { return e.Err }
-
 type InvalidSymbolError struct {
 	Sym string
 	Err error