using jquery

$("#get").click(function() {
    var ytl = $("#youtubeLink").val();
    var yti = ytl.match(/(?:https?:\/{2})?(?:w{3}\.)?youtu(?:be)?\.(?:com|be)(?:\/watch\?v=|\/)([^\s&]+)/);
    $("#msg").html("<h1>YouTube Thumbnail</h1><p><img src=\"http://i3.ytimg.com/vi/" + yti[1] + "/maxresdefault.jpg\" class=\"image\" /></p><p><img src=\"http://i3.ytimg.com/vi/" + yti[1] + "/hqdefault.jpg\" class=\"image\" /></p><h2>Image URL:</h2><br /><input name=\"imageURL\" id=\"copyimageURL\" class=\"text\" size=\"70\" type=\"text\" value=\"http://i3.ytimg.com/vi/" + yti[1] + "/maxresdefault.jpg\" onclick=\"this.select()\" readonly /><br /><input name=\"imageURL\" id=\"copyimageURL\" class=\"text\" size=\"70\" type=\"text\" value=\"http://i3.ytimg.com/vi/" + yti[1] + "/hqdefault.jpg\" onclick=\"this.select()\" readonly /><p>Right/Ctrl Click the image to download.  Highlight the URL above and copy to your clipboard.</p>");

});


$("#copyimageURL").click(function() {
    $(this).select();
});

using js

// https://youtu.be/UHQPdP8qgrk

var ytl  = "https://youtu.be/UHQPdP8qgrk"
var regexp = /(?:https?:\/{2})?(?:w{3}\.)?youtu(?:be)?\.(?:com|be)(?:\/watch\?v=|\/)([^\s&]+)/
ytt = ytl.match(regexp)
ytid = ytt[1]
// console.log(ytid)
yttmax[1]= "http://i3.ytimg.com/vi/" + ytid[1] + "/maxresdefault.jpg"
ytthq = "http://i3.ytimg.com/vi/" + ytid[1] + "/hqdefault.jpg"

// console.log(yttmax)