diff --git a/example.ts b/example.ts
index 7c42052..d56e755 100755
--- a/example.ts
+++ b/example.ts
@@ -1,10 +1,11 @@
 #!/bin/env -S deno run
 
-import { extr } from "./mod.ts"
+import { def } from "./mod.ts"
 
-const date = ["month", " ", "day", " ", "h", ":", "m", ":", "s"] as const;
-const service = extr("", ...date, " ", "device", " ", "service", "[", "pid", "]: ", "line");
-const kernel_ = extr("", ...date, " ", "device", " kernel: ", "line");
+
+const date = "{{month}} {{day}} {{h}}:{{m}}:{{s}}" as const;
+const service = def(`${date} {{device}} {{service}}[{{pid}}]: {{line}}`);
+const kernel_ = def(`${date} {{device}} kernel: {{line}}`);
 
 const lines = [
     "août 06 09:25:18 navis systemd[1]: Finished Load Kernel Module configfs.",
diff --git a/lib/lib.ts b/lib/lib.ts
index 0a75dbe..a3337b5 100644
--- a/lib/lib.ts
+++ b/lib/lib.ts
@@ -1,6 +1,7 @@
 import { Rule } from "./rule.ts";
 import { PairElts, StructOfArr } from "./types.ts";
 import { Extractor } from "./extractor.ts"
+import { split_once } from "./utils.ts";
 
 export function extr<A extends string[]>(...args: A): Extractor<StructOfArr<PairElts<A>, string>> {
     const seps = [] as string[];
@@ -18,7 +19,7 @@ export function extr<A extends string[]>(...args: A): Extractor<StructOfArr<Pair
     return new Extractor(text => rule.get(text))
 }
 
-Deno.test("types", async () => {
+Deno.test("extr_types", async () => {
     const { assertEquals } = await import("https://deno.land/std@0.224.0/assert/assert_equals.ts");
     const vars = extr()
         .or(extr("bonjour, ", 'animal', ", poilu"))
@@ -26,3 +27,34 @@ Deno.test("types", async () => {
         .get("bonjour, le chêne, feuillu");
     assertEquals({ vars }, { vars: { "arbre": "le chêne" } })
 })
+
+type ArrForDef<D extends string> =
+    D extends "" ? [] :
+    D extends `${infer W extends string}{{${infer V extends string}}}${infer R extends string}` ? [W, V, ...ArrForDef<R>] :
+    D extends `${infer W extends string}` ? [W] :
+    never;
+
+export function def<D extends string>(def: D): Extractor<StructOfArr<PairElts<ArrForDef<D>>, string>> {
+    const result = [] as string[];
+    let rest = def as string;
+    while (true) {
+        const [word, var_rest] = split_once(rest, "{{");
+        if (var_rest === null) break;
+        const [var_, rest_] = split_once(var_rest, "}}");
+        if (rest_ === null) throw new Error(`Invalid rule near '${rest}'`);
+        result.push(word, var_);
+        rest = rest_;
+    }
+    if (rest.length > 0) result.push(rest);
+    const args = result as ArrForDef<D>;
+    return extr(...args);
+}
+
+Deno.test("def_types", async () => {
+    const { assertEquals } = await import("https://deno.land/std@0.224.0/assert/assert_equals.ts");
+    const vars = extr()
+        .or(def("bonjour, {{animal}}, poilu"))
+        .or(def("bonjour, {{arbre}}, feuillu"))
+        .get("bonjour, le chêne, feuillu");
+    assertEquals({ vars }, { vars: { "arbre": "le chêne" } })
+})
diff --git a/lib/rule.ts b/lib/rule.ts
index 3f950a7..c663126 100644
--- a/lib/rule.ts
+++ b/lib/rule.ts
@@ -1,5 +1,5 @@
 import { StructOfArr } from "./types.ts";
-import { zip } from "./utils.ts";
+import { split_once, zip } from "./utils.ts";
 
 export class Rule<V extends string[]> {
     private separators: ({ kind: "word", word: string } | { kind: "end" })[];
@@ -32,11 +32,3 @@ export class Rule<V extends string[]> {
         return result as StructOfArr<V, string>;
     }
 }
-
-function split_once(text: string, separator: string) {
-    const cursor = text.indexOf(separator);
-    if (cursor === -1) return [text, null] as const;
-    const left = text.slice(0, cursor);
-    const right = text.slice(cursor + separator.length);
-    return [left, right] as const;
-}
diff --git a/lib/utils.ts b/lib/utils.ts
index b59e86a..41c59d5 100644
--- a/lib/utils.ts
+++ b/lib/utils.ts
@@ -12,3 +12,11 @@ export function next<T>(iterator: Iterator<T>) {
     if (result === undefined) return null;
     else return result as T;
 }
+
+export function split_once(text: string, separator: string) {
+    const cursor = text.indexOf(separator);
+    if (cursor === -1) return [text, null] as const;
+    const left = text.slice(0, cursor);
+    const right = text.slice(cursor + separator.length);
+    return [left, right] as const;
+}
diff --git a/mod.ts b/mod.ts
index b7107de..541ae2d 100644
--- a/mod.ts
+++ b/mod.ts
@@ -1 +1 @@
-export { extr } from "./lib/lib.ts"
+export { extr, def } from "./lib/lib.ts"