POSIX File Permission Octal Regex for Python

What this pattern does

This page provides a lightweight, single-purpose regular expression for matching posix file permission octal, ported and verified for Python. A rigorously tested regex reduces debugging time and protects your application from edge-case failures. The snippet below is ready to drop into your Python project — whether you're validating in a Django view, a FastAPI endpoint, or a standalone data processing script.

Python Implementation

# POSIX File Permission Octal
# ReDoS-checked | RegexVault — Dev & Systems > File Paths

import re

posix_file_permission_octal_pattern = re.compile(r'^([0-7]?)([0-7]{3})$')

def validate_posix_file_permission_octal(value: str) -> bool:
    return bool(posix_file_permission_octal_pattern.fullmatch(value))

# Example
print(validate_posix_file_permission_octal("755"))  # True

Test Cases

When to use this pattern

This pattern is drawn from the Dev & Systems > File Paths category and has been checked for ReDoS risk. That matters for Python developers because particularly important in Python web servers where CPU-bound regex operations can stall concurrent request handling. 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.