Regular Expression for Specific Hostnames

2 min read 09-11-2024

Regular Expression for Specific Hostnames

Regular expressions (regex) are powerful tools used to match patterns in strings, including hostnames. Crafting a regex for specific hostnames involves understanding the structure and rules that govern how hostnames are formed.

Understanding Hostnames

A valid hostname typically adheres to the following rules:

It can contain letters (a-z, A-Z), digits (0-9), and hyphens (-).
It must start and end with a letter or digit.
It can have up to 253 characters total.
Each label (section separated by dots) must be between 1 and 63 characters.

Common Hostname Patterns

Before we dive into regex patterns, let’s consider some common scenarios. For example, you may want to match:

A specific hostname: e.g., example.com
Subdomains: e.g., *.example.com (matches any subdomain)
Multiple specific hostnames: e.g., example.com and test.example.com

Regular Expression Examples

1. Matching a Specific Hostname

To match exactly example.com:

^example\.com$

^ asserts the start of the string.
example matches the literal string.
\.com matches .com (the dot is escaped).
$ asserts the end of the string.

2. Matching Any Subdomain of a Specific Domain

To match any subdomain under example.com:

^(?:[a-zA-Z0-9-]+\.)?example\.com$

(?:...) is a non-capturing group.
[a-zA-Z0-9-]+ matches one or more letters, digits, or hyphens.
\. matches a literal dot.
The ? makes the entire non-capturing group optional, allowing for the base domain to match without a subdomain.

3. Matching Multiple Specific Hostnames

To match either example.com or test.example.com:

^(example\.com|test\.example\.com)$

The pipe (|) operator acts as an OR, allowing you to match one of the specified patterns.

4. Matching a Range of Hostnames

If you want to match any hostname that starts with dev. and ends with example.com:

^dev\.[a-zA-Z0-9-]+\.example\.com$

dev\. specifies that the hostname must start with dev..
[a-zA-Z0-9-]+ matches the middle part of the hostname.

Conclusion

Regular expressions are a versatile way to validate and match specific hostnames. By understanding the structure of hostnames and using the appropriate regex patterns, you can effectively manage hostname validation and matching tasks in various applications. Always remember to test your regex patterns with different hostname examples to ensure they work as intended!