Custom regexp external issues (#17624)

* Implement custom regular expression for external issue tracking.

Signed-off-by: Alexander Beyn <malex@fatelectrons.org>

* Fix syntax/style

* Update repo.go

* Set metas['regexp']

* gofmt

* fix some tests

* fix more tests

* refactor frontend

* use LRU cache for regexp

* Update modules/markup/html_internal_test.go

Co-authored-by: Alexander Beyn <malex@fatelectrons.org>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/regexplru/regexplru.go

@@ -0,0 +1,45 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package regexplru
+
+import (
+	"regexp"
+
+	"code.gitea.io/gitea/modules/log"
+
+	lru "github.com/hashicorp/golang-lru"
+)
+
+var lruCache *lru.Cache
+
+func init() {
+	var err error
+	lruCache, err = lru.New(1000)
+	if err != nil {
+		log.Fatal("failed to new LRU cache, err: %v", err)
+	}
+}
+
+// GetCompiled works like regexp.Compile, the compiled expr or error is stored in LRU cache
+func GetCompiled(expr string) (r *regexp.Regexp, err error) {
+	v, ok := lruCache.Get(expr)
+	if !ok {
+		r, err = regexp.Compile(expr)
+		if err != nil {
+			lruCache.Add(expr, err)
+			return nil, err
+		}
+		lruCache.Add(expr, r)
+	} else {
+		r, ok = v.(*regexp.Regexp)
+		if !ok {
+			if err, ok = v.(error); ok {
+				return nil, err
+			}
+			panic("impossible")
+		}
+	}
+	return r, nil
+}

modules/regexplru/regexplru_test.go

@@ -0,0 +1,27 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package regexplru
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestRegexpLru(t *testing.T) {
+	r, err := GetCompiled("a")
+	assert.NoError(t, err)
+	assert.True(t, r.MatchString("a"))
+
+	r, err = GetCompiled("a")
+	assert.NoError(t, err)
+	assert.True(t, r.MatchString("a"))
+
+	assert.EqualValues(t, 1, lruCache.Len())
+
+	_, err = GetCompiled("(")
+	assert.Error(t, err)
+	assert.EqualValues(t, 2, lruCache.Len())
+}