regex for usernae code example

Example 1: regex for username

let username = '';
username = username.replace(/\s/g,'_');
username = username.replace(/\-/g,'.');
username = username.match(/[a-zA-Z0-9\.\s]+/g).join('_');

Example 2: username regex

^(?=[a-zA-Z0-9._]{8,20}$)(?!.*[_.]{2})[^_.].*[^_.]$