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

A valid matching string is "ensibey". A non-matching string is "-invalid-name".

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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GitHub Username Regex

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

Use regex.test(): const regex = /^[a-zA-Z0-9](?:[a-zA-Z0-9]|-(?=[a-zA-Z0-9])){0,38}$/; const isValid = regex.test(inputString);

Strictly complies with GitHub username rules: max 39 chars, alphanumeric, single internal hyphens, no consecutive hyphens, cannot start or end with a hyphen.

Regular Expression Pattern:

/^[a-zA-Z0-9](?:[a-zA-Z0-9]|-(?=[a-zA-Z0-9])){0,38}$/

Valid Match Example:

ensibey

Invalid / Non-Match Example:

-invalid-name

Regex Syntax Breakdown & Explanation

•^[a-zA-Z0-9] : Must begin with alphanumeric character
•-(?=[a-zA-Z0-9]) : Hyphens permitted only if followed immediately by alphanumeric (no consecutive hyphens)
•{0,38}$ : Total length capped at 39 characters

Implementation in Popular Languages

JavaScript / TypeScript

const regex = /^[a-zA-Z0-9](?:[a-zA-Z0-9]|-(?=[a-zA-Z0-9])){0,38}$/;
const isValid = regex.test("ensibey");
console.log(isValid); // true

Python (re)

import re

pattern = r"^[a-zA-Z0-9](?:[a-zA-Z0-9]|-(?=[a-zA-Z0-9])){0,38}$"
match = re.match(pattern, "ensibey")
print(bool(match)) # True

Go (regexp)

package main
import (
  "fmt"
  "regexp"
)

func main() {
  re := regexp.MustCompile(`^[a-zA-Z0-9](?:[a-zA-Z0-9]|-(?=[a-zA-Z0-9])){0,38}$`)
  fmt.Println(re.MatchString("ensibey"))
}

Frequently Asked Questions

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

The regular expression pattern is /^[a-zA-Z0-9](?:[a-zA-Z0-9]|-(?=[a-zA-Z0-9])){0,38}$/. Strictly complies with GitHub username rules: max 39 chars, alphanumeric, single internal hyphens, no consecutive hyphens, cannot start or end with a hyphen.