What is an example of a matching and non-matching string?

A valid matching string is "https://staging.example.com". A non-matching string is "example.com".

Is this regex safe from ReDoS (Catastrophic Backtracking)?

Yes, this pattern is optimized for linear execution time and safe for browser-side validation without locking up the UI thread.

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Subdomain Extraction Regex

Q: How do I test this regex in JavaScript and TypeScript?

Use regex.test(): const regex = /^(?:https?:\/\/)?([a-zA-Z0-9-]+)\.[a-zA-Z0-9-]+\.[a-zA-Z]{2,}/i; const isValid = regex.test(inputString);

Captures and isolates the third-level subdomain prefix from a full hostname or URL.

Regular Expression Pattern:

/^(?:https?:\/\/)?([a-zA-Z0-9-]+)\.[a-zA-Z0-9-]+\.[a-zA-Z]{2,}/i

Valid Match Example:

https://staging.example.com

Invalid / Non-Match Example:

example.com

Regex Syntax Breakdown & Explanation

•(?:https?:\/\/)? : Disregards optional protocol schema
•([a-zA-Z0-9-]+) : First capturing group isolates the leading subdomain label
•\.[a-zA-Z0-9-]+\.[a-zA-Z]{2,} : Confirms presence of root domain and TLD

Implementation in Popular Languages

JavaScript / TypeScript

const regex = /^(?:https?:\/\/)?([a-zA-Z0-9-]+)\.[a-zA-Z0-9-]+\.[a-zA-Z]{2,}/i;
const isValid = regex.test("https://staging.example.com");
console.log(isValid); // true

Python (re)

import re

pattern = r"^(?:https?:\/\/)?([a-zA-Z0-9-]+)\.[a-zA-Z0-9-]+\.[a-zA-Z]{2,}"
match = re.match(pattern, "https://staging.example.com")
print(bool(match)) # True

Go (regexp)

package main
import (
  "fmt"
  "regexp"
)

func main() {
  re := regexp.MustCompile(`^(?:https?:\/\/)?([a-zA-Z0-9-]+)\.[a-zA-Z0-9-]+\.[a-zA-Z]{2,}`)
  fmt.Println(re.MatchString("https://staging.example.com"))
}

Frequently Asked Questions

Everything you need to know regarding specifications, syntax, and security best practices.

The regular expression pattern is /^(?:https?:\/\/)?([a-zA-Z0-9-]+)\.[a-zA-Z0-9-]+\.[a-zA-Z]{2,}/i. Captures and isolates the third-level subdomain prefix from a full hostname or URL.