diff --git a/xeval/mod.ts b/xeval/mod.ts
new file mode 100644
index 000000000000..ceca25a799dd
--- /dev/null
+++ b/xeval/mod.ts
@@ -0,0 +1,170 @@
+import { parse } from "../flags/mod.ts";
+const { Buffer, EOF, args, exit, stdin, writeAll } = Deno;
+type Reader = Deno.Reader;
+
+/* eslint-disable-next-line max-len */
+// See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncFunction.
+const AsyncFunction = Object.getPrototypeOf(async function(): Promise<void> {})
+  .constructor;
+
+const HELP_MSG = `Deno xeval
+
+USAGE:
+  deno -A https://deno.land/std/xeval/mod.ts [OPTIONS] <code>
+
+OPTIONS:
+  -d, --delim <delim>       Set delimiter, defaults to newline
+  -I, --replvar <replvar>   Set variable name to be used in eval, defaults to $
+
+ARGS:
+  <code>`;
+
+export type XevalFunc = (v: string) => void;
+
+export interface XevalOptions {
+  delimiter?: string;
+}
+
+const DEFAULT_DELIMITER = "\n";
+
+// Generate longest proper prefix which is also suffix array.
+function createLPS(pat: Uint8Array): Uint8Array {
+  const lps = new Uint8Array(pat.length);
+  lps[0] = 0;
+  let prefixEnd = 0;
+  let i = 1;
+  while (i < lps.length) {
+    if (pat[i] == pat[prefixEnd]) {
+      prefixEnd++;
+      lps[i] = prefixEnd;
+      i++;
+    } else if (prefixEnd === 0) {
+      lps[i] = 0;
+      i++;
+    } else {
+      prefixEnd = pat[prefixEnd - 1];
+    }
+  }
+  return lps;
+}
+
+// TODO(kevinkassimo): Move this utility somewhere public in deno_std.
+// Import from there once doable.
+// Read from reader until EOF and emit string chunks separated
+// by the given delimiter.
+async function* chunks(
+  reader: Reader,
+  delim: string
+): AsyncIterableIterator<string> {
+  const encoder = new TextEncoder();
+  const decoder = new TextDecoder();
+  // Avoid unicode problems
+  const delimArr = encoder.encode(delim);
+  const delimLen = delimArr.length;
+  const delimLPS = createLPS(delimArr);
+
+  let inputBuffer = new Buffer();
+  const inspectArr = new Uint8Array(Math.max(1024, delimLen + 1));
+
+  // Modified KMP
+  let inspectIndex = 0;
+  let matchIndex = 0;
+  while (true) {
+    let result = await reader.read(inspectArr);
+    if (result === EOF) {
+      // Yield last chunk.
+      const lastChunk = inputBuffer.toString();
+      yield lastChunk;
+      return;
+    }
+    if ((result as number) < 0) {
+      // Discard all remaining and silently fail.
+      return;
+    }
+    let sliceRead = inspectArr.subarray(0, result as number);
+    await writeAll(inputBuffer, sliceRead);
+
+    let sliceToProcess = inputBuffer.bytes();
+    while (inspectIndex < sliceToProcess.length) {
+      if (sliceToProcess[inspectIndex] === delimArr[matchIndex]) {
+        inspectIndex++;
+        matchIndex++;
+        if (matchIndex === delimLen) {
+          // Full match
+          const matchEnd = inspectIndex - delimLen;
+          const readyBytes = sliceToProcess.subarray(0, matchEnd);
+          // Copy
+          const pendingBytes = sliceToProcess.slice(inspectIndex);
+          const readyChunk = decoder.decode(readyBytes);
+          yield readyChunk;
+          // Reset match, different from KMP.
+          sliceToProcess = pendingBytes;
+          inspectIndex = 0;
+          matchIndex = 0;
+        }
+      } else {
+        if (matchIndex === 0) {
+          inspectIndex++;
+        } else {
+          matchIndex = delimLPS[matchIndex - 1];
+        }
+      }
+    }
+    // Keep inspectIndex and matchIndex.
+    inputBuffer = new Buffer(sliceToProcess);
+  }
+}
+
+export async function xeval(
+  reader: Reader,
+  xevalFunc: XevalFunc,
+  { delimiter = DEFAULT_DELIMITER }: XevalOptions = {}
+): Promise<void> {
+  for await (const chunk of chunks(reader, delimiter)) {
+    // Ignore empty chunks.
+    if (chunk.length > 0) {
+      await xevalFunc(chunk);
+    }
+  }
+}
+
+async function main(): Promise<void> {
+  const parsedArgs = parse(args.slice(1), {
+    boolean: ["help"],
+    string: ["delim", "replvar"],
+    alias: {
+      delim: ["d"],
+      replvar: ["I"],
+      help: ["h"]
+    },
+    default: {
+      delim: DEFAULT_DELIMITER,
+      replvar: "$"
+    }
+  });
+  if (parsedArgs._.length != 1) {
+    console.error(HELP_MSG);
+    exit(1);
+  }
+  if (parsedArgs.help) {
+    return console.log(HELP_MSG);
+  }
+
+  const delimiter = parsedArgs.delim;
+  const replVar = parsedArgs.replvar;
+  const code = parsedArgs._[0];
+
+  // new AsyncFunction()'s error message for this particular case isn't great.
+  if (!replVar.match(/^[_$A-z][_$A-z0-9]*$/)) {
+    console.error(`Bad replvar identifier: "${replVar}"`);
+    exit(1);
+  }
+
+  const xEvalFunc = new AsyncFunction(replVar, code);
+
+  await xeval(stdin, xEvalFunc, { delimiter });
+}
+
+if (import.meta.main) {
+  main();
+}
diff --git a/xeval/test.ts b/xeval/test.ts
new file mode 100644
index 000000000000..6b3e64ff4067
--- /dev/null
+++ b/xeval/test.ts
@@ -0,0 +1,49 @@
+import { xeval } from "./mod.ts";
+import { stringsReader } from "../io/util.ts";
+import { decode, encode } from "../strings/mod.ts";
+import { assertEquals, assertStrContains } from "../testing/asserts.ts";
+import { test } from "../testing/mod.ts";
+const { execPath, run } = Deno;
+
+test(async function xevalSuccess(): Promise<void> {
+  const chunks: string[] = [];
+  await xeval(stringsReader("a\nb\nc"), ($): number => chunks.push($));
+  assertEquals(chunks, ["a", "b", "c"]);
+});
+
+test(async function xevalDelimiter(): Promise<void> {
+  const chunks: string[] = [];
+  await xeval(stringsReader("!MADMADAMADAM!"), ($): number => chunks.push($), {
+    delimiter: "MADAM"
+  });
+  assertEquals(chunks, ["!MAD", "ADAM!"]);
+});
+
+// https://github.com/denoland/deno/issues/2861
+// TODO: Use the URL constructor here when it's fixed.
+const modTsUrl = import.meta.url.replace(/test.ts$/, "mod.ts");
+
+test(async function xevalCliReplvar(): Promise<void> {
+  const p = run({
+    args: [execPath(), modTsUrl, "--replvar=abc", "console.log(abc)"],
+    stdin: "piped",
+    stdout: "piped",
+    stderr: "null"
+  });
+  await p.stdin!.write(encode("hello"));
+  await p.stdin!.close();
+  assertEquals(await p.status(), { code: 0, success: true });
+  assertEquals(decode(await p.output()), "hello\n");
+});
+
+test(async function xevalCliSyntaxError(): Promise<void> {
+  const p = run({
+    args: [execPath(), modTsUrl, "("],
+    stdin: "null",
+    stdout: "piped",
+    stderr: "piped"
+  });
+  assertEquals(await p.status(), { code: 1, success: false });
+  assertEquals(decode(await p.output()), "");
+  assertStrContains(decode(await p.stderrOutput()), "Uncaught SyntaxError");
+});