Validate a subdomain with JavaScript, inspired from https://gist.github.com/stuartbain/7212385

validateSubdomain.js

const MIN_LENGTH = 5;
const MAX_LENGTH = 63;
const ALPHA_NUMERIC_REGEX = /^[a-z][a-z\-]*[a-z0-9]*$/;
const START_END_HYPHEN_REGEX = /\A[^-].*[^-]\z/i;
const reservedNames = ['www', 'ftp', 'mail', 'pop', 'smtp', 'admin', 'ssl', 'sftp'];

const validateSubdomain = subdomain => {
    //if is reserved...
    if (reservedNames.includes(subdomain))
        throw new Error('cannot be a reserved name');

    //if is too small or too big...
    if (subdomain.length < MIN_LENGTH || subdomain.length > MAX_LENGTH)
        throw new Error(`must have between ${MIN_LENGTH} and ${MAX_LENGTH} characters`);


    //if subdomain is started/ended with hyphen or is not alpha numeric
    if (!ALPHA_NUMERIC_REGEX.test(subdomain))
        throw new Error(
            (subdomain.indexOf('-') === 0 || subdomain.indexOf('-') === (subdomain.length - 1)) ?
                'cannot start or end with a hyphen' :
                'must be alphanumeric (or hyphen)'
        );

    return true;
}