JSFiddle - React, Tailwind, and code Playground

CSS

[\w\-\.]+\/..\/|\:\/\/[\w\-\.\/]+http

JavaScript

var magicUrlCombine = function(a,b){
   return (a + b).replace(/[\w\-\.]+\/..\/|\:\/\/[\w\-\.\/]+http/g,'');
}

var a = 'http://example.com/some/path/';
var b = '../other/path/';
var c = magicUrlCombine(a,b);

document.write(c + '<br/>');


var a = 'http://example.com/some/path/';
var b = '../other/path/../path2/';
var c = magicUrlCombine(a,b);

document.write(c + '<br/>');


var a = 'http://example.com/some/path/../cat/dog/fish/';
var b = '../other/path/../path2/';
var c = magicUrlCombine(a,b);

document.write(c + '<br/>');

a = 'http://example.com/some/path/';
b = 'http://pink-unicorns.com/some/other/path/';
c = magicUrlCombine(a,b);

document.write(c + '<br/>');


a = 'http://example.com/some/path/../nowherer/';
b = 'http://pink-unicorns.com/some/other/path/../somethingelse/';
c = magicUrlCombine(a,b);

document.write(c + '<br/>');