Domain Name Validation Regex

Validate domain names with proper subdomain and TLD structure using regex pattern for lowercase alphanumeric domains.

Pattern Breakdown

^([a-z0-9]+(-[a-z0-9]+)*\.)+[a-z]{2,}$

Components

Detailed Breakdown

• ([a-z0-9]+(-[a-z0-9]+)*\.)+ - Domain parts (one or more):
  • [a-z0-9]+ - One or more lowercase letters or digits
  • (-[a-z0-9]+)* - Zero or more hyphen-separated segments
  • \. - Literal dot separator
  • + - One or more domain parts (allows subdomains)
• [a-z]{2,} - TLD: two or more lowercase letters

Examples

Valid:

• example.com
• subdomain.example.com
• multi-level.example.com
• test-domain.co.uk
• example123.org
• my-site.io

Invalid:

• example (missing TLD)
• example.c (TLD too short)
• EXAMPLE.COM (uppercase not allowed)
• example..com (double dot)
• -example.com (starts with hyphen)
• example-.com (ends with hyphen)
• example.com. (trailing dot)

Implementation

JavaScript

const domainRegex = /^([a-z0-9]+(-[a-z0-9]+)*\.)+[a-z]{2,}$/;
domainRegex.test('example.com'); // true
domainRegex.test('subdomain.example.com'); // true
domainRegex.test('test-domain.co.uk'); // true
domainRegex.test('example'); // false (missing TLD)
domainRegex.test('EXAMPLE.COM'); // false (uppercase)

Python

import re
domain_regex = r'^([a-z0-9]+(-[a-z0-9]+)*\.)+[a-z]{2,}$'
bool(re.match(domain_regex, 'example.com'))  # True
bool(re.match(domain_regex, 'subdomain.example.com'))  # True
bool(re.match(domain_regex, 'example'))  # False (missing TLD)

Go

domainRegex := regexp.MustCompile(`^([a-z0-9]+(-[a-z0-9]+)*\.)+[a-z]{2,}$`)
domainRegex.MatchString("example.com") // true
domainRegex.MatchString("subdomain.example.com") // true
domainRegex.MatchString("example") // false (missing TLD)

Limitations

1. Lowercase only: Only accepts lowercase letters
2. No underscore: Doesn't allow underscores in domain names
3. No multi-part TLD validation: Doesn't specifically validate .co.uk format
4. No length limits: Doesn't enforce DNS length limits (253 chars total, 63 per label)
5. No IDN support: Doesn't support internationalized domain names
6. Format only: Validates format, not actual DNS existence

When to Use

• Domain name format validation
• DNS-related applications
• Subdomain validation
• When you need strict format checking
• Domain input forms

For production, consider:

• Supporting uppercase (case-insensitive)
• Adding length validation (max 253 chars, 63 per label)
• Validating against known TLD lists
• Supporting internationalized domain names
• Combining with DNS lookup for existence verification