diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..bdaf4b8
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,5 @@
+module github.com/haya14busa/xtag
+
+go 1.12
+
+require github.com/haya14busa/go-versionsort v0.0.0-20190326014014-5b3a40d6070a
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..d01780d
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,2 @@
+github.com/haya14busa/go-versionsort v0.0.0-20190326014014-5b3a40d6070a h1:C75TMjl9nVoJt0B2KcbO6qESAF3lG4vVYgDXoxotWrg=
+github.com/haya14busa/go-versionsort v0.0.0-20190326014014-5b3a40d6070a/go.mod h1:LUfyxWIo5I3AzeZxF5+/wAUM5sz5ZibEh/Ck3dZrN50=
diff --git a/xtag.go b/xtag.go
new file mode 100644
index 0000000..1f9ddc2
--- /dev/null
+++ b/xtag.go
@@ -0,0 +1,35 @@
+package xtag
+
+import (
+	"fmt"
+	"regexp"
+	"strings"
+
+	"github.com/haya14busa/go-versionsort"
+)
+
+// FindLatest finds latest tag matched with give xtag (e.g. v1.2.x).
+func FindLatest(xtag string, tags []string) (string, error) {
+	if !strings.HasSuffix(xtag, "x") {
+		return "", fmt.Errorf("xtag must ends with 'x': %q", xtag)
+	}
+	target := "x"
+	if strings.HasSuffix(xtag, ".x") {
+		target = `\.x`
+	}
+	pattern := strings.Replace(regexp.QuoteMeta(xtag), target, `[0-9.]*`, -1)
+	re, err := regexp.Compile(pattern)
+	if err != nil {
+		return "", err
+	}
+	latest := ""
+	for _, tag := range tags {
+		if re.MatchString(tag) && versionsort.Less(latest, tag) {
+			latest = tag
+		}
+	}
+	if latest != "" {
+		return latest, nil
+	}
+	return "", fmt.Errorf("latest tag not found: %q", xtag)
+}
diff --git a/xtag_test.go b/xtag_test.go
new file mode 100644
index 0000000..83bb3da
--- /dev/null
+++ b/xtag_test.go
@@ -0,0 +1,28 @@
+package xtag
+
+import "testing"
+
+func TestFindLatest(t *testing.T) {
+	tests := []struct {
+		xtag string
+		tags []string
+		want string
+	}{
+		{"v1.x", []string{"v1.0", "v1.5", "v1.4", "v2.0"}, "v1.5"},
+		{"1.x", []string{"1.0", "1.5", "1.4", "2.0"}, "1.5"},
+		{"v2.x", []string{"v1.0", "v2.0.0", "v3.0"}, "v2.0.0"},
+		{"v1.2.x", []string{"v1.0", "v2.0.0", "v1.2.1", "v1.2.3"}, "v1.2.3"},
+		{"vx", []string{"v1.0", "v2.0.0", "v1.2.1"}, "v2.0.0"},
+		{"x", []string{"1.0", "2.0.0", "1.2.1"}, "2.0.0"},
+	}
+	for _, tt := range tests {
+		got, err := FindLatest(tt.xtag, tt.tags)
+		if err != nil {
+			t.Errorf("%s: got error: %v", tt.xtag, err)
+			continue
+		}
+		if got != tt.want {
+			t.Errorf("FindLatest(%q, %s) = %q, want %q", tt.xtag, tt.tags, got, tt.want)
+		}
+	}
+}