README

UrlLinker is a PHP module for converting plain text snippets to HTML, and any web addresses in the text into HTML hyperlinks.

Installing

1. Download the lib using [Composer](https://getcomposer.org/):

   ` composer require kwi/urllinker `

2. In your PHP file require Composer's autoloader:

   ``` <?php

   require_once __DIR__.'/vendor/autoload.php'; ```

Usage (procedural API)

print(htmlEscapeAndLinkUrls($text));

For a longer example, see UrlLinker-example.php.

UrlLinker assumes plain text input, and returns HTML. If your input is already HTML, but it contains URLs that have not been marked up, UrlLinker can handle that as well:

print(linkUrlsInTrustedHtml($html));

Warning: The latter function must only be used on trusted input, as rendering HTML provided by a malicious user can lead to system compromise through cross-site scripting. The htmlEscapeAndLinkUrls function, on the other hand, can safely be used on untrusted input. (You can remove existing tags from untrusted input via PHP's strip_tags function.)

Usage (object oriented API)

``` $urlLinker = new KwiUrlLinker();

$urlLinker->linkUrlsAndEscapeHtml($text);

$urlLinker->linkUrlsInTrustedHtml($html); ```

In your functions, you can depend on UrlLinkerInterface:

``` class Example {

private $urlLinker;

public function __construct(KwiUrlLinkerInterface $urlLinker) {

$this->urlLinker = $urlLinker;

}

public function doStuff($text) {

// do sth with $text…

return $this->urlLinker->linkUrlsAndEscapeHtml($text);

}

}

You can configure different options for parsing URLs by passing them into UrlLinker's constructor:

``` // Ftp addresses like "ftp://example.com" will be allowed: $urlLinker = new KwiUrlLinker(true);

// Uppercase URL schemes like "HTTP://exmaple.com" will be allowed: $urlLinker = new KwiUrlLinker(false, true); ```

Recognized addresses

• Web addresses
  • Recognized URL schemes: "http" and "https"
    • The http:// prefix is optional.
    • Support for additional schemes, e.g. "ftp", can easily be added by tweaking $rexScheme.
    • The scheme must be written in lower case. This requirement can be lifted by adding an i (the PCRE_CASELESS modifier) to $rexUrlLinker.
  • Hosts may be specified using domain names or IPv4 addresses.
    • IPv6 addresses are not supported.
  • Port numbers are allowed.
  • Internationalized Resource Identifiers (IRIs) are allowed. Note that the job of converting IRIs to URIs is left to the user's browser.
  • To reduce false positives, UrlLinker verifies that the top-level domain is on the official IANA list of valid TLDs.
    • UrlLinker is updated from time to time as the TLD list is expanded.
    • In the future, this approach may collapse under ICANN's ill-advised new policy of selling arbitrary TLDs for large amounts of cash, but for now it is an effective method of rejecting invalid URLs.
    • Internationalized top-level domain names must be written in Punycode in order to be recognized.
    • If you need to support unqualified domain names, such as localhost, you may disable the TLD check by 1) replacing + with * in the $rexDomain value and 2) replacing the if statement line beneath the "Check that the TLD is valid" comment with if (true). This is obviously a quick-and-dirty hack, and may cause false positives.
• Email addresses
  • Supports the full range of commonly used address formats, including "plus addresses" (as popularized by Gmail).
  • Does not recognized the more obscure address variants that are allowed by the RFCs but never seen in practice.
  • Simplistic spam protection: The at-sign is converted to a HTML entity, foiling naive email address harvesters.
• Addresses are recognized correctly in normal sentence contexts. For instance, in "Visit stackoverflow.com.", the final period is not part of the URL.
• User input is properly sanitized to prevent cross-site scripting (XSS), and ampersands in URLs are correctly escaped as & (this does not apply to the linkUrlsInTrustedHtml function, which assumes its input to be valid HTML).

Tests

Unit tests are written using [PHPUnit](https://phpunit.de).

` $ cd PATH_TO_URL_LINKER $ composer install $ phpunit `

Background

A Stackoverflow.com question prompted me to consider the difficulty of this task. Initially, it seemed easy, but like an itch you just have to scratch, I kept coming back to it, to fix just one more little thing.

Feel free to upvote my answer if you find this code useful.

There's also a C# implementation by Antoine Sottiau.

Public Domain Dedication

To the extent possible under law, the author has waived all copyright and related or neighboring rights to UrlLinker.

For more information see: http://creativecommons.org/publicdomain/zero/1.0/