URL-Encoded Form Data Body Regex for JavaScript

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching url-encoded form data body, 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

// URL-Encoded Form Data Body
// ReDoS-safe | RegexVault — Web & Network > Misc

const urlencodedFormDataBodyRegex = /^[a-zA-Z0-9%+\-_.~!$'()*,;:@\/?]+(?:=[a-zA-Z0-9%+\-_.~!$'()*,;:@\/?]*)?(?:&[a-zA-Z0-9%+\-_.~!$'()*,;:@\/?]+(?:=[a-zA-Z0-9%+\-_.~!$'()*,;:@\/?]*)?)*$/;

function validateUrlencodedFormDataBody(input: string): boolean {
  return urlencodedFormDataBodyRegex.test(input);
}

// Example
console.log(validateUrlencodedFormDataBody("key=value")); // 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

Spaces are encoded as + (form encoding) or %20 (percent encoding). Both are valid. The pattern accepts key-only parameters (no = sign) and empty values (key=).