Improve validation of urls

[yanokwa]

In UrlUtils, the way we check URLs is like so:

try {
  new URL(url);
  return true;
} catch (MalformedURLException e) {
  return false;
}

This fails with things that are probably not URLs, like https:appspot.com which technically is a URL, but you know, it's not a URL folks are likely to enter. I'd think we should do better.

We could search for the perfect URL regex, but it doesn't exist. We can add UrlValidator but it seems strange to add a library for just this.

I propose we define a URL to be a string that starts with http:// or https:// and has a least one character. So in regex, ^(http|https)://.+$

[dcbriccetti]

A small improvement: ^https?:\/\/.+$

[nribeka]

Are we just check the format of the url or do you want to actually check the url?

If we want to check the url whether it is valid and can be used, we probably want to issue HEAD call and see what we're getting? Or this is not gonna too much for folks in the field (consuming their data allocation)? This probably gonna help them making sure that they're actually entering correct server location.

[yanokwa]

I don't think we want to actually check the URL, we just want to check the format. Checking the URL requires the server to be up (duh) but there are times when you want to pre-configure the devices first.

[ykayacan]

Hello! Do you consider about using Android's built in WEB_URL pattern matcher?

Example utility code is below.

public static boolean isUrl(String url) {
Pattern p = Patterns.WEB_URL;
Matcher m = p.matcher(url.toLowerCase());
return m.matches();
}

[yanokwa]

@krialix Yes, I considered it. For one, it depends on a fixed list of top level domains which change quickly and so you get different behavior on different versions of Android. See http://stackoverflow.com/a/29771086. Have you found this tradeoff acceptable in your use of Patterns.WEB_URL? Have you confirmed that it reports URLs like https:appspot.com as invalid?

[ykayacan]

@yanokwa Yes it reports invalid. We use this pattern in our app and worked well for now, but I like to test it for other Android versions (currently we tested it on Android 7.1.1).

Moreover, if it is not valid for older Android versions maybe you can consider to add URL_VALID_DOMAIN from Regex class which is actively developed by Twitter.

[yanokwa]

I think a simple regex is strictly better because it:

1. Has the same behavior across all version of Android.
2. Does not require an external library.

@lognaturel does not feel strongly about this, so I'm going to proceed with a PR.