Networking

Hostname (Single Label, RFC 1123)

Validate a single-label hostname per RFC 1123: 1–63 chars, letters/digits/hyphens, can't start or end with hyphen.

Available in

JavaScript / ECMAScript code

jsJavaScript

const re = new RegExp("^[a-zA-Z0-9](?:[a-zA-Z0-9\\-]{0,61}[a-zA-Z0-9])?$", "");
const input = "web-01";
const matches = [...input.matchAll(re)];
console.log(matches.map(m => m[0]));

Uses `String.prototype.matchAll` for global iteration (Node 12+ / all modern browsers).

Python (re) code

pyPython

import re

pattern = re.compile(r"^[a-zA-Z0-9](?:[a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])?$")
input_text = "web-01"
for m in pattern.finditer(input_text):
    print(m.group(0))

Stdlib `re` module — no third-party dependency. Works on Python 3.6+.

Go (RE2) code

goGo

package main

import (
	"fmt"
	"regexp"
)

func main() {
	re := regexp.MustCompile(`^[a-zA-Z0-9](?:[a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])?$`)
	input := `web-01`
	for _, match := range re.FindAllString(input, -1) {
		fmt.Println(match)
	}
}

Uses `regexp.MustCompile` (panics on bad patterns at startup) and `FindAllString` for all matches.

Pattern

regexengine-agnostic

^[a-zA-Z0-9](?:[a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])?$

Raw source: ^[a-zA-Z0-9](?:[a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])?$

How it works

^[a-zA-Z0-9] requires the first character to be a letter or digit. (?:[a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])? optionally matches up to 61 more characters (letters, digits, hyphens) followed by a letter/digit ending — preventing trailing hyphens. Combined max length: 63 characters per RFC 1123.