Data URI (Base64) Regex for JavaScript

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching data uri (base64), 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

// Data URI (Base64)
// ReDoS-safe | RegexVault — Web & Network > URL

const dataUriBase64Regex = /^data:([a-zA-Z][a-zA-Z0-9!#$&\-^_]{0,59}\/[a-zA-Z][a-zA-Z0-9!#$&\-^_+.]{0,59});base64,([A-Za-z0-9+\/]{4})*([A-Za-z0-9+\/]{2}==|[A-Za-z0-9+\/]{3}=)?$/;

function validateDataUriBase64(input: string): boolean {
  return dataUriBase64Regex.test(input);
}

// Example
console.log(validateDataUriBase64("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg==")); // true

Test Cases

When to use this pattern

This pattern is drawn from the Web & Network > URL 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

Base64 padding (= or ==) is structurally validated. The MIME type is captured in group 1. Maximum practical data URI size is ~2MB for browser compatibility.