Base64 Encoded String Regex for JavaScript

What this pattern does

This page provides a well-structured, multi-part regular expression for matching base64 encoded string, ported and verified for JavaScript. A rigorously tested regex reduces debugging time and protects your application from edge-case failures. The snippet below is ready to drop into your JavaScript project — whether you're validating in an Express middleware, a Next.js API route, or a client-side form.

Javascript Implementation

// Base64 Encoded String
// ReDoS-safe | RegexVault — Web & Network > Misc

const base64EncodedStringRegex = /^(?:[A-Za-z0-9+\/]{4})*(?:[A-Za-z0-9+\/]{2}==|[A-Za-z0-9+\/]{3}=)?$/;

function validateBase64EncodedString(input: string): boolean {
  return base64EncodedStringRegex.test(input);
}

// Example
console.log(validateBase64EncodedString("SGVsbG8sIFdvcmxkIQ==")); // true

Test Cases

When to use this pattern

This pattern is drawn from the Web & Network > Misc category and carries a ReDoS-safe certification. That matters for JavaScript developers because especially critical in long-running Node.js event loops where a ReDoS vulnerability can block the entire process. RegexVault audits patterns against known backtracking attack vectors, ensuring you have the necessary context before using this regex in a high-stakes production environment.

Common Pitfalls

Technical Notes

Empty string is valid base64 (encodes empty byte string). This pattern validates standard base64 — for URL-safe base64, replace + with \- and / with _.