JS Regex url validation

I believe the other answer will reject some valid url's (like domain names in uppercase or long sub-domains) and allow some invalid ones (like http://www.-example-.com or www.%@&.com). I tried to take into account a number of additional url syntax rules (without getting into internationalisation).

function isUrlValid(userInput) {
    var regexQuery = "^(https?://)?(www\\.)?([-a-z0-9]{1,63}\\.)*?[a-z0-9][-a-z0-9]{0,61}[a-z0-9]\\.[a-z]{2,6}(/[-\\w@\\+\\.~#\\?&/=%]*)?$";
    var url = new RegExp(regexQuery,"i");
    return url.test(userInput);
}
var input = ["https://o.sub-domain.example.com/foo/bar?foo=bar&boo=far#a%20b",
             "HTTP://EX-AMPLE.COM",
             "example.c",
             "example-.com"];
for (var i in input) document.write(isUrlValid(input[i]) + ": " + input[i] + "<br>");

To also allow IP addresses and port numbers, the regex is:

"^(https?://)?(((www\\.)?([-a-z0-9]{1,63}\\.)*?[a-z0-9][-a-z‌​0-9]{0,61}[a-z0-9]\\‌​.[a-z]{2,6})|((\\d{1‌​,3}\\.){3}\\d{1,3}))‌​(:\\d{2,4})?(/[-\\w@‌​\\+\\.~#\\?&/=%]*)?$‌​"  

To also allow query strings without a slash between the domain name and the question mark (which is theoretically not allowed, but works in most real-life situations), the regex is:

"^(https?://)?(((www\\.)?([-a-z0-9]{1,63}\\.)*?[a-z0-9][-a-z‌​0-9]{0,61}[a-z0-9]\\‌​.[a‌​-z]{2,6})|((\\d‌​{1,3}\\.){3}\\d{1,3}‌​))(:\\d{2,4})?((/|\\‌​?)[-\\w@\\+\\.~#\\?&‌​/=%]*)?$"

To also make sure that every % is followed by a hex number, the regex is:

"^(https?://)?(((www\\.)?([-a-z0-9]{1,63}\\.)*?[a-z0-9][-a-z‌​0-9]{0,61}[a-z0-9]\\‌​.[a-z]{2,6})|((\\d{1‌​,3}\\.){3}\\d{1,3}))‌​(:\\d{2,4})?((/|\\?)‌​(((%[0-9a-f]{2})|[-\‌​\w@\\+\\.~#\\?&/=])*‌​))?$"

(Note: as John Wu mentioned in a comment, there are valid single-letter domains).

Try this code.

function CheckURL(fieldId, alertMessage) {
    var url = fieldId.value;
    if(url !== "")
    {
        if (url.match(/(http(s)?:\/\/.)?(www\.)?[-a-zA-Z0-9@:%._\+~#=]{2,256}\.[a-z]{2,6}\b([-a-zA-Z0-9@:%_\+.~#?&//=]*)/g) !== null)
            return true;
        else {
            alert(alertMessage);
            fieldId.focus();
            return false;
        }
    }
}

var website = document.getElementById('Website');
if (!CheckURL(website, "Enter a valid website address")) {
    return false;
}

Actually, this question needs a powerful regex and the following code is not very hard to understand, please see below(ES6 - TypeScript):

const isValidUrl = (url: string): boolean => {
  const urlRegex = /^((http(s?)?):\/\/)?([wW]{3}\.)?[a-zA-Z0-9\-.]+\.[a-zA-Z]{2,}(\.[a-zA-Z]{2,})?$/g;
  const result = url.match(urlRegex);

  return result !== null;
};

I change the function to Match + make a change here with the slashes and its work: (http(s)?://.)

The fixed function:

function isUrlValid(userInput) {
    var res = userInput.match(/(http(s)?:\/\/.)?(www\.)?[-a-zA-Z0-9@:%._\+~#=]{2,256}\.[a-z]{2,6}\b([-a-zA-Z0-9@:%_\+.~#?&//=]*)/g);
    if(res == null)
        return false;
    else
        return true;
}