How to check if a string is a valid URL

Question

WordPress provides a helpful function called is_email() which checks if a given email address is valid. Is there a similar function available to check if a URL is valid?

I tried is_url() but that was just wishful thinking on my part.

Ref: http://codex.wordpress.org/Function_Reference/is_email

Answer 1

Use the native php function Filter Validator

if (filter_var($url, FILTER_VALIDATE_URL) === FALSE) {
    die('Not a valid URL');
}

Answer 2

I know this is an old post, but for anyone visiting it's also worth considering the WP functions esc_url() and esc_url_raw(), the latter being safe for databases entries etc, as it does not encode entities. esc_url() does encode entities and so is good for display to users.

In the source you can see that esc_url() checks for a whitelist of allowed protocols and structure and so avoids some of the vulnerabilities of FILTER_VALIDATE_URL noted by the link posed by @tobltobs.

Answer 3

I found wp_http_validate_url quite convenient to check if a string in valid URL or not while working on my project.

Take a reference from here: https://developer.wordpress.org/reference/functions/wp_http_validate_url/

For example:

$val = 'http://somevalidurl.com';
if ( wp_http_validate_url( $val ) ) {

    // It's valid URL;

} else {

    // It's NOT valid URL;

}

It returns the URL itself if it's valid,else false.

Answer 4

In my opinion is better to use wp_http_validate_url.

Example 1:

filter_var( '//website.com', FILTER_VALIDATE_URL )

Return false.

Example 2:

wp_http_validate_url( '//website.com' )

Return url.

1. When you don't know is website use https or http protocol, it's better to use '//'.
2. wp_http_validate_url is improved by WP developers (is more specific).
3. Theoretically you can use some hooks (filters & actions) for wp_http_validate_url. You can't use hooks for filter_var.

Ref: https://developer.wordpress.org/reference/functions/wp_http_validate_url/

Ref: https://www.php.net/manual/en/function.filter-var.php

Answer 5

Let's discuss our options for validating URLs in WordPress starting with the obvious ones.

1. filter_var() with the FILTER_VALIDATE_URL filter in PHP doesn't work for internationalized domain names containing non-ASCII characters such as http://스타벅스코리아.com. It also has some other issues, as discussed by David Müller, like accepting http://example.com/"><script>alert("xss")</script> as valid URL even though it is clearly malicious.

2. wp_http_validate_url() in WordPress may do a better job even though it was intended specifically for using in HTTP API and not for the general validation of URLs. But it still doesn't solve the two specific issues discussed earlier.

3. preg_match() in PHP may work for you or it may not. Writing regular expressions forecasting every possible scenario is not always the most ideal solution and getting it universally right may not be possible from time to time.

4. esc_url_raw() in WordPress is not intended for validating URLs but it seems to be doing a better job than the others. It will work for non-ASCII characters as well as solve the other issues discussed earlier. This function will basically sanitize any string for saving in the WordPress database as a URL by removing or altering any characters that are either invalid or malicious. And therefore, if the sanitized version isn't the same as the unsanitized version, it can be pretty confidently said that the URL isn't valid and maybe most importantly not safe.

Here is how the URL verification can be done with the esc_url_raw() function by also taking advantage of the strtolower() function to make the verification case insensitive.

function validateUrl($url) {
    if ( strtolower(esc_url_raw($url)) === strtolower($url) ) {
        return $url;
    } else {
        return false;
    }
}

Answer 6

To ensure user-submitted URL is valid and safe to store and later display, I suggest

esc_url_raw($url) === $url

esc_url_raw(), like @orionrush mentioned above, sanitizes the URL by removing anything invalid or malicious from it. So, if there string has nothing invalid or malicious, then it's ok.

Eg

if( esc_url_raw($url) === $url ) {
   // URL is valid. Use it...
} else {
   // URL is invalid or malicious. Don't use it...
}

I wrote a longer blog post on that here: https://cmljnelson.wordpress.com/2018/08/31/url-validation-in-wordpress/

Answer 7

is_email(), actually do not check if an email address is valid, only if it is conforming to spec. The email I gladly supply for many sites that show pointless interest in my email address is a@b.com which is conforming address but it is unlikely to have anyone using it.

Checking out urls is best done by pinging them. You can try to check conformance, but in theory there are very little limitation to what an encoded url can be.