Edit in JSFiddle

console.log("Halli, halli, hallo".match(/hall./gi)); 

console.log("Chatarsis og katarsis".match(/[kc]atarsis/gi));

console.log("Catarsis og katarsis".replace(/[kc]atarsis/gi,"Nonsens"));

let test = "Åh nej, en ny sætning med en eller to prøvelser, Øv!";
console.log(test.match(/[a-zA-Z_0-9æøåÆØÅ]+/gi));


let test2 = "Åh nej. I Nur mit Böse Früchte und Gemüse. Øv!";
console.log(test2.match(/(?<=mit)[^]+/gi));


console.log(test.match(/(?<=^|[^\wÀ-ÿ])[\wÀ-ÿ]{2}(?=$|[^\wÀ-ÿ])/gi));

let nonWordchars = "[^a-zA-Z_0-9\\u00C0-\\u00FF]";
let wordChars = "[a-zA-Z_0-9\\u00C0-\\u00FF]";
let boundaryStart = "(?<=^|" + nonWordchars + ")";
let boundaryEnd = "(?=$|" + nonWordchars + ")";
let pattern =  new RegExp(boundaryStart + wordChars + "{2}" + boundaryEnd, "gi");
console.log(test.match(pattern));


const html = document.getElementById("markup").innerHTML;

const urlPattern = /(?<=\<a[^>]* href=")([^"]*)/ig;    
console.log(html.match(urlPattern));

const scriptPattern = /(?<=\<script[^>]* src=")([^"]*)/ig;    
console.log(html.match(scriptPattern));



<div id=markup><script async defer 
     src="https://www.example.com/default.js"></script>
<script src="/www.microsoft.com/test.js"></script>
<a class="linkclass" href="https://euromind.com/default.html"></a> 
<a href="http://www.euromind.com"></a></div>