The one I use is slightly more advanced, can't copy it for some strange reason, but I use a service that checks for a valid domain first

Same, but my version uses a different approach. It takes the selected (or entered via prompt) text, strip spaces and opens the sci-hub in a separate tab.

  javascript:  Qr = window.getSelection ? window.getSelection().toString() : document.selection.createRange().text; if (Qr) {     void(window.open('https://sci-hub.se/' + String(Qr).replace(/^\s+|\s+$/g, ''))); } else {     void(Qr = prompt('Enter URL/DOI to sci-hub it', ''));     location.href = 'https://sci-hub.se/' + String(Qr).replace(/^\s+|\s+$/g, ''); }