dotenv File Line Regex for JavaScript

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching dotenv file line, 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

// dotenv File Line
// ReDoS-safe | RegexVault — Dev & Systems > Environment

const dotenvFileLineRegex = /^(?:export\s+)?([A-Za-z_][A-Za-z0-9_]{0,254})\s*=\s*(?:"([^"]*)"|'([^']*)'|([^#\n]*?))?\s*(?:#.*)?$/;

function validateDotenvFileLine(input: string): boolean {
  return dotenvFileLineRegex.test(input);
}

// Example
console.log(validateDotenvFileLine("NODE_ENV=production")); // true

Test Cases

When to use this pattern

This pattern is drawn from the Dev & Systems > Environment 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

Groups: 1=key, 2=double-quoted value, 3=single-quoted value, 4=unquoted value. Comment lines starting with # fail this pattern — skip them explicitly. Use a dedicated .env parser for production.