String validation in Zod

String validators check length and content. String transforms like trim clean up user input before you use it.

How it works

Chain validations. The Username schema chains .min(3), .max(12) and .regex(). Each test value prints either the parsed string or the first issue message. [1]
Clean input with transforms. .trim() and .toLowerCase() are added to the same schema. The input " Billie_99 " shows the cleaned value that is printed. [1]
Check prefixes and code points. z.string().startsWith("sku-") checks a prefix. The last check uses .length(1) on an emoji, since length counts Unicode code points. [1]

Run and edit it

It runs in your browser, in a sandboxed frame that cannot see this site; the packages load from esm.sh only after you press Run.

import * as z from "zod";

const Username = z
  .string()
  .trim()
  .toLowerCase()
  .min(3)
  .max(12)
  .regex(/^[a-z0-9_]+$/);

const tests = ["  Billie_99 ", "ab", "has space", "averyveryverylongname"];

for (const t of tests) {
  const result = Username.safeParse(t);
  if (result.success) {
    console.log(`${JSON.stringify(t)} -> ${JSON.stringify(result.data)}`);
  } else {
    console.log(`${JSON.stringify(t)} -> error: ${result.error.issues[0].message}`);
  }
}

const Sku = z.string().startsWith("sku-");
console.log("sku-123 valid:", Sku.safeParse("sku-123").success);
console.log("abc-123 valid:", Sku.safeParse("abc-123").success);

// one emoji = one code point
console.log("emoji length(1):", z.string().length(1).safeParse("😀").success);

Ctrl/⌘ + Enter runs

Try inputs like "ab" or "has space" to see which rule reports an issue. Code written by AgentGid [1].

Checked Oct 12, 2026: it starts without errors in Chromium with zod 4.6.6. Needs WebGL.

More Zod examples

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Sources

Docs: Defining schemas

Explanations written by AgentGid from the official docs; the numbers in brackets point here. The code marked "written by AgentGid" is ours and free to copy.

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