POSIX File Permission Octal Regex for JavaScript

What this pattern does

This page provides a lightweight, single-purpose regular expression for matching posix file permission octal, 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

// POSIX File Permission Octal
// ReDoS-safe | RegexVault — Dev & Systems > File Paths

const posixFilePermissionOctalRegex = /^([0-7]?)([0-7]{3})$/;

function validatePosixFilePermissionOctal(input: string): boolean {
  return posixFilePermissionOctalRegex.test(input);
}

// Example
console.log(validatePosixFilePermissionOctal("755")); // true

Test Cases

When to use this pattern

This pattern is drawn from the Dev & Systems > File Paths 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

Optional leading digit: 4=setuid, 2=setgid, 1=sticky. Three digits: owner/group/other (read=4, write=2, execute=1). 1777 = /tmp permissions (sticky). 4755 = setuid root (dangerous). Group 1 = special bits, group 2 = rwx permissions.