Make `user-content-* ` consistent with github (#26388)

Fix #26367
Related #19745

Thanks @lazyky for providing  test cases

modules/markup/common/footnote.go

@@ -29,17 +29,12 @@ func CleanValue(value []byte) []byte {
 	value = bytes.TrimSpace(value)
 	rs := bytes.Runes(value)
 	result := make([]rune, 0, len(rs))
-	needsDash := false
 	for _, r := range rs {
-		switch {
+		if unicode.IsLetter(r) || unicode.IsNumber(r) || r == '_' || r == '-' {
-		case unicode.IsLetter(r) || unicode.IsNumber(r) || r == '_':
-			if needsDash && len(result) > 0 {
-				result = append(result, '-')
-			}
-			needsDash = false
 			result = append(result, unicode.ToLower(r))
-		default:
+		}
-			needsDash = true
+		if unicode.IsSpace(r) {
+			result = append(result, '-')
 		}
 	}
 	return []byte(string(result))

modules/markup/common/footnote_test.go

@@ -0,0 +1,60 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+package common
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCleanValue(t *testing.T) {
+	tests := []struct {
+		param  string
+		expect string
+	}{
+		// Github behavior test cases
+		{"", ""},
+		{"test(0)", "test0"},
+		{"test!1", "test1"},
+		{"test:2", "test2"},
+		{"test*3", "test3"},
+		{"test！4", "test4"},
+		{"test：5", "test5"},
+		{"test*6", "test6"},
+		{"test：6 a", "test6-a"},
+		{"test：6 !b", "test6-b"},
+		{"test：ad # df", "testad--df"},
+		{"test：ad #23 df 2*/*", "testad-23-df-2"},
+		{"test：ad 23 df 2*/*", "testad-23-df-2"},
+		{"test：ad # 23 df 2*/*", "testad--23-df-2"},
+		{"Anchors in Markdown", "anchors-in-markdown"},
+		{"a_b_c", "a_b_c"},
+		{"a-b-c", "a-b-c"},
+		{"a-b-c----", "a-b-c----"},
+		{"test：6a", "test6a"},
+		{"test：a6", "testa6"},
+		{"tes a a   a  a", "tes-a-a---a--a"},
+		{"  tes a a   a  a  ", "tes-a-a---a--a"},
+		{"Header with \"double quotes\"", "header-with-double-quotes"},
+		{"Placeholder to force scrolling on link's click", "placeholder-to-force-scrolling-on-links-click"},
+		{"tes（）", "tes"},
+		{"tes（0）", "tes0"},
+		{"tes{0}", "tes0"},
+		{"tes[0]", "tes0"},
+		{"test【0】", "test0"},
+		{"tes…@a", "tesa"},
+		{"tes￥& a", "tes-a"},
+		{"tes= a", "tes-a"},
+		{"tes|a", "tesa"},
+		{"tes\\a", "tesa"},
+		{"tes/a", "tesa"},
+		{"a啊啊b", "a啊啊b"},
+		{"c🤔️🤔️d", "cd"},
+		{"a⚡a", "aa"},
+		{"e.~f", "ef"},
+	}
+	for _, test := range tests {
+		assert.Equal(t, []byte(test.expect), CleanValue([]byte(test.param)), test.param)
+	}
+}