diff --git a/CHANGELOG.md b/CHANGELOG.md
index a416f36..6f40c20 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,10 @@
 
 ## [Unreleased]
 
+## [0.9.0] - 2024-01-16
+### Changed
+- Add source type parameter to `Parser`.
+
 ## [0.8.0] - 2023-06-24
 ### Added
 - Added `Parser.prototype.apply`
diff --git a/examples/__snapshots__/script.test.ts.snap b/examples/__snapshots__/script.test.ts.snap
index 509b9e6..75ffd7e 100644
--- a/examples/__snapshots__/script.test.ts.snap
+++ b/examples/__snapshots__/script.test.ts.snap
@@ -3,14 +3,14 @@
 exports[`script 1`] = `
 {
   "last": null,
-  "stats": [
+  "stmts": [
     {
       "body": {
         "last": {
           "elements": [],
           "type": "Tuple",
         },
-        "stats": [
+        "stmts": [
           {
             "expr": {
               "arguments": [
diff --git a/examples/json.test.ts b/examples/json.test.ts
index 9d092e9..f499715 100644
--- a/examples/json.test.ts
+++ b/examples/json.test.ts
@@ -20,11 +20,8 @@ describe("JSON", () => {
         });
     });
 
-    test("number parsing to fail..", () => {
-        expect(jsonParser.parse("00")).toHaveProperty("success", false);
-        expect(jsonParser.parse("- 0")).toHaveProperty("success", false);
-        expect(jsonParser.parse("0.")).toHaveProperty("success", false);
-        expect(jsonParser.parse(".0")).toHaveProperty("success", false);
+    test.each(["00", "- 0", "0.", ".0"])("%o is invalid json.", n => {
+        expect(jsonParser.parse(n)).toHaveProperty("success", false);
     });
 
     test.each([
diff --git a/examples/s-expression.test.ts b/examples/s-expression.test.ts
new file mode 100644
index 0000000..691b2d4
--- /dev/null
+++ b/examples/s-expression.test.ts
@@ -0,0 +1,22 @@
+import { describe, expect, test } from "@jest/globals";
+import { EOI } from "../src";
+import { SExpression } from "./s-expression";
+
+describe("S-expression", () => {
+    test("Hello world!", () => {
+        const source = '(print "Hello world!")';
+        expect(SExpression.skip(EOI).parse(source)).toEqual({
+            success: true,
+            index: expect.any(Number),
+            value: ["print", '"Hello world!"'],
+        });
+    });
+    test("quote list", () => {
+        const source = "'(1 2 3 4)";
+        expect(SExpression.skip(EOI).parse(source)).toEqual({
+            success: true,
+            index: expect.any(Number),
+            value: ["quote", "1", "2", "3", "4"],
+        });
+    });
+});
diff --git a/examples/s-expression.ts b/examples/s-expression.ts
new file mode 100644
index 0000000..5c1a950
--- /dev/null
+++ b/examples/s-expression.ts
@@ -0,0 +1,16 @@
+import { type Parser, choice, el, lazy, many, regex } from "../src";
+
+export type SExpression = string | readonly SExpression[];
+
+const list = lazy(() => SExpression)
+    .apply(many)
+    .between(el("("), el(")"));
+
+export const SExpression: Parser<SExpression, string> = choice([
+    el("'")
+        .option()
+        .and(list)
+        .map(([quote, list]) => (quote ? ["quote", ...list] : list)),
+    regex(/"([^"\\]|\\.)*"/),
+    regex(/[^\s()"]+/),
+]).between(regex(/\s*/));
diff --git a/examples/script.test.ts b/examples/script.test.ts
index fee9100..2ef6b09 100644
--- a/examples/script.test.ts
+++ b/examples/script.test.ts
@@ -1,9 +1,9 @@
 import { describe, expect, test } from "@jest/globals";
-import { expr, stat } from "./script";
+import { expr, stmt } from "./script";
 
-describe("stat", () => {
+describe("stmt", () => {
     test("Let", () => {
-        const result = stat.parse("let hoge = 0;");
+        const result = stmt.parse("let hoge = 0;");
         expect(result.success && result.value).toEqual({
             type: "Let",
             name: "hoge",
@@ -12,17 +12,17 @@ describe("stat", () => {
     });
 
     test("DefFn", () => {
-        const result = stat.parse("fn main(arg) {};");
+        const result = stmt.parse("fn main(arg) {};");
         expect(result.success && result.value).toEqual({
             type: "DefFn",
             name: "main",
             params: ["arg"],
-            body: { type: "Block", stats: [], last: null },
+            body: { type: "Block", stmts: [], last: null },
         });
     });
 
     test("Return", () => {
-        const result = stat.parse("return;");
+        const result = stmt.parse("return;");
         expect(result.success && result.value).toEqual({
             type: "Return",
             body: null,
@@ -30,23 +30,23 @@ describe("stat", () => {
     });
 
     test("While", () => {
-        const result = stat.parse("while (false) {};");
+        const result = stmt.parse("while (false) {};");
         expect(result.success && result.value).toEqual({
             type: "While",
             test: { type: "Bool", value: false },
-            body: { type: "Block", stats: [], last: null },
+            body: { type: "Block", stmts: [], last: null },
         });
     });
 
     test("Break", () => {
-        const result = stat.parse("break;");
+        const result = stmt.parse("break;");
         expect(result.success && result.value).toEqual({
             type: "Break",
         });
     });
 
     test("Expr", () => {
-        const result = stat.parse("0;");
+        const result = stmt.parse("0;");
         expect(result.success && result.value).toEqual({
             type: "Expr",
             expr: { type: "Number", value: 0 },
@@ -127,15 +127,15 @@ describe("expr", () => {
             const result = expr.parse("{}");
             expect(result.success && result.value).toEqual({
                 type: "Block",
-                stats: [],
+                stmts: [],
                 last: null,
             });
         });
-        test("stats", () => {
+        test("stmts", () => {
             const result = expr.parse("{ 0; }");
             expect(result.success && result.value).toEqual({
                 type: "Block",
-                stats: [{ type: "Expr", expr: { type: "Number", value: 0 } }],
+                stmts: [{ type: "Expr", expr: { type: "Number", value: 0 } }],
                 last: null,
             });
         });
@@ -143,7 +143,7 @@ describe("expr", () => {
             const result = expr.parse("{ 0 }");
             expect(result.success && result.value).toEqual({
                 type: "Block",
-                stats: [],
+                stmts: [],
                 last: { type: "Number", value: 0 },
             });
         });
@@ -155,7 +155,7 @@ describe("expr", () => {
             expect(result.success && result.value).toEqual({
                 type: "If",
                 test: { type: "Bool", value: true },
-                then: { type: "Block", stats: [], last: null },
+                then: { type: "Block", stmts: [], last: null },
                 else: null,
             });
         });
diff --git a/examples/script.ts b/examples/script.ts
index 6099d05..8df6d6e 100644
--- a/examples/script.ts
+++ b/examples/script.ts
@@ -5,7 +5,7 @@ export type Expr =
     | { type: "Number"; value: number }
     | { type: "String"; value: string }
     | { type: "Tuple"; elements: readonly Expr[] }
-    | { type: "Block"; stats: Stat[]; last: Expr | null }
+    | { type: "Block"; stmts: Stmt[]; last: Expr | null }
     | { type: "If"; test: Expr; then: Expr; else: Expr | null }
     | { type: "Ident"; name: string }
     | { type: "Call"; callee: Expr; arguments: readonly Expr[] }
@@ -39,7 +39,7 @@ const keyword = (keyword: string): P.Parser<unknown, string> => {
 
 const Ident = P.regex(/\w+/).map(name => ({ type: "Ident", name }) satisfies Expr);
 
-export type Stat =
+export type Stmt =
     | { type: "Let"; name: string; init: Expr }
     | { type: "DefFn"; name: string; params: readonly string[]; body: Expr }
     | { type: "Return"; body: Expr | null }
@@ -50,7 +50,7 @@ export type Stat =
 const Let = keyword("let")
     .then(Ident.between(ws))
     .skip(P.el("="))
-    .andMap(expr, ({ name }, init): Stat => ({ type: "Let", name, init }));
+    .andMap(expr, ({ name }, init): Stmt => ({ type: "Let", name, init }));
 
 const DefFn = P.seq([
     keyword("fn").then(Ident.between(ws)),
@@ -60,7 +60,7 @@ const DefFn = P.seq([
         .between(P.el("("), P.el(")"))
         .map(nodes => nodes.map(node => node.name)),
     expr,
-]).map<Stat>(([{ name }, params, body]) => ({
+]).map<Stmt>(([{ name }, params, body]) => ({
     type: "DefFn",
     name,
     params,
@@ -70,18 +70,18 @@ const DefFn = P.seq([
 const Return = keyword("return")
     .then(expr.option(null))
     .skip(ws)
-    .map<Stat>(body => ({ type: "Return", body }));
+    .map<Stmt>(body => ({ type: "Return", body }));
 
 const While = keyword("while")
     .skip(ws)
     .then(expr.between(P.el("("), P.el(")")))
-    .andMap(expr, (test, body): Stat => ({ type: "While", test, body }));
+    .andMap(expr, (test, body): Stmt => ({ type: "While", test, body }));
 
-const Break = keyword("break").return<Stat>({ type: "Break" }).skip(ws);
+const Break = keyword("break").return<Stmt>({ type: "Break" }).skip(ws);
 
-const Expr = expr.map<Stat>(expr => ({ type: "Expr", expr }));
+const Expr = expr.map<Stmt>(expr => ({ type: "Expr", expr }));
 
-export const stat: P.Parser<Stat, string> = P.choice([
+export const stmt: P.Parser<Stmt, string> = P.choice([
     Let,
     DefFn,
     Return,
@@ -126,10 +126,10 @@ const Tuple = expr
     .between(P.el("("), P.el(")"))
     .map(elements => ({ type: "Tuple", elements }) satisfies Expr);
 
-const Block = stat
+const Block = stmt
     .apply(P.many)
-    .andMap(expr.option(null), (stats, last) => {
-        return { type: "Block", stats, last } satisfies Expr;
+    .andMap(expr.option(null), (stmts, last) => {
+        return { type: "Block", stmts, last } satisfies Expr;
     })
     .skip(ws)
     .between(P.el("{"), P.el("}"));
diff --git a/package-lock.json b/package-lock.json
index 62e8aa3..180e0a0 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "parsea",
-  "version": "0.8.0",
+  "version": "0.9.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "parsea",
-      "version": "0.8.0",
+      "version": "0.9.0",
       "license": "MIT",
       "dependencies": {
         "emnorst": "^1.0.0-next.2"
diff --git a/package.json b/package.json
index 2ccf051..2733bc4 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "parsea",
-  "version": "0.8.0",
+  "version": "0.9.0",
   "description": "parser combinator library for parsing ArrayLike.",
   "type": "module",
   "main": "dist/index.cjs",
diff --git a/src/combinator.ts b/src/combinator.ts
index 3d7eac2..1319d60 100644
--- a/src/combinator.ts
+++ b/src/combinator.ts
@@ -7,13 +7,11 @@ import { type ParseState, updateState } from "./state";
  * Delays variable references until the parser runs.
  */
 export const lazy = <T, S>(getParser: () => Parser<T, S>): Parser<T, S> => {
-    let parser: Parser<T, S>;
-    return new Parser((state, context) => {
-        if (parser == null) {
-            parser = getParser();
-        }
-        return parser.run(state, context);
+    const lazyParser: Parser<T, S> = new Parser((state, context) => {
+        // @ts-expect-error readonly
+        return (lazyParser.run = getParser().run)(state, context);
     });
+    return lazyParser;
 };
 
 export const notFollowedBy = <S>(parser: Parser<unknown, S>): Parser<unknown, S> =>
diff --git a/src/index.ts b/src/index.ts
index f1b3713..a342e4f 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,5 +1,5 @@
 export * from "./combinator";
 export * from "./do";
-export type { Config, ParseResult, Parsed, Parser } from "./internal";
+export type { Config, ParseResult, Parsed, Parser, Source } from "./internal";
 export * from "./primitive";
 export * from "./string";
diff --git a/src/string.ts b/src/string.ts
index d5e8fa5..e20b139 100644
--- a/src/string.ts
+++ b/src/string.ts
@@ -110,14 +110,19 @@ export const ANY_CHAR = /* @__PURE__ */ new Parser<string, string>((state, conte
 });
 
 export const regexGroup = (re: RegExp): Parser<RegExpExecArray, string> => {
-    const fixedRegex = new RegExp(`^(?:${re.source})`, re.flags.replace("g", ""));
+    let flags = re.flags.replace("g", "");
+    if (!re.sticky) {
+        flags += "y";
+    }
+    const fixedRegex = new RegExp(re, flags);
 
     return new Parser((state, context) => {
         if (typeof context.src !== "string") {
             context.addError(state.i);
             return null;
         }
-        const matchResult = fixedRegex.exec(context.src.slice(state.i));
+        fixedRegex.lastIndex = state.i;
+        const matchResult = fixedRegex.exec(context.src);
         if (matchResult === null) {
             context.addError(state.i);
             return null;
@@ -135,10 +140,11 @@ export const regex: {
         defaultValue: T,
     ): Parser<string | T, string>;
 } = (re: RegExp, groupId: number | string = 0, defaultValue?: undefined) =>
-    regexGroup(re).map(
-        matchResult =>
-            (typeof groupId === "number"
+    regexGroup(re).map(matchResult => {
+        const groupValue =
+            typeof groupId === "number"
                 ? matchResult[groupId]
                 : // biome-ignore lint/style/noNonNullAssertion: overrideのため
-                  matchResult.groups?.[groupId]!) ?? defaultValue,
-    );
+                  matchResult.groups?.[groupId]!;
+        return groupValue ?? defaultValue;
+    });