Question

Preg_Replace entire URLs based on domain

Asked by: Tomeeboy

I'm a bit rusty with my regex (okay, really rusty) and I need to put together a small set of filters to weed out banned or unwanted links from a forum system.  Basically, I want to be able to specify a domain, such as website.com, and then filter a message for any URLs to that domain (whether in raw form or BBcode).  If found, the entire URL is removed or replaced (preferably replaced with a message or warning of some kind).  This would be done at the time of posting, and not on display.

Example:

"Hey everybody, go to http://www.bannedwebsite.com/id=23525&page=234234 for great deals on ..."

If I was filtering for "bannedwebsite.com", it would catch this and change the text to:

"Hey everybody, go to <spam link removed> for great deals on ..."

I think that in many cases, the URLs will also be inside of BBCode, so I need to check for these as well.  Examples of the BBcode used:

[url]http://www.bannedwebsite.com[/url]

and

[url=http://www.bannedwebsite.com]Click Me[/url]

I need some examples to achieve this, if anyone would be so kind as to help out.  I'm sure it's pretty simple to do, but my head is killing me just thinking about it at the moment :)  Thanks in advance!

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Asked On
2006-12-20 at 17:05:25ID22099988
Tags

preg_replace

,

domain

Topic

PHP Scripting Language

Participating Experts
3
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Comments
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Answers

 

by: soapergemPosted on 2006-12-20 at 17:23:47ID: 18177821

Here you go. I created a function and provided an example. All you need to do is feed it the text you want to filter and an array filled with the domains you wish to ban.

    <?php
   
    function ban_domain($text, $array_of_domains)
    {
        $domains = str_replace('.', '\\.', implode('|', $array_of_domains));
        $search = '@(\\[url\\]|\\[url=)?https?://.*?(?:' . $domains . ').*?/?(\\[/url\\]|\\].*?\\[/url\\])?@si';
        $replace = '&lt;spam link removed&gt;';
        return preg_replace($search, $replace, $text);
    }
   
    $text = 'Example. http://www.bad.ru/ Example. [url]http://www.bad2.com/[/url] Example. [url=http://www.bad.ru/]Spam[/url] Example.';
    echo ban_domain($text, array('bad.ru', 'bad2.com'));
   
    ?>

 

by: soapergemPosted on 2006-12-20 at 17:26:19ID: 18177833

Actually, to work a little better, change the $search line to this:

$search = '@(\\[url\\]|\\[url=)?https?://.*?(?:' . $domains . ')[^\\s]*(\\[/url\\]|\\].*?\\[/url\\])?@si';

 

by: soapergemPosted on 2006-12-20 at 17:30:04ID: 18177845

Wait....change it to this:

$search = '@(\\[url\\]|\\[url=)?https?://.*?(?:' . $domains . ').*?(\\[/url\\]|\\].*?\\[/url\\])?[^\\s]*@si';

I promise that's my last change. (Can you tell I do a lot of trial-and-error?) So the full context would be something like this:

    <?php
   
    function ban_domain($text, $array_of_domains)
    {
        $domains = str_replace('.', '\\.', implode('|', $array_of_domains));
        $search = '@(\\[url\\]|\\[url=)?https?://.*?(?:' . $domains . ').*?(\\[/url\\]|\\].*?\\[/url\\])?[^\\s]*@si';
        $replace = '&lt;spam link removed&gt;';
        return preg_replace($search, $replace, $text);
    }
    $text1 = 'Hey everybody, go to http://www.bannedwebsite.com/id=23525&page=234234 for great deals on ...';
    $text2 = 'Hey everybody, go to [url]http://www.bannedwebsite.com[/url]';
    $text3 = '[url=http://www.bannedwebsite.com]Click Me[/url]';
    echo ban_domain($text1, array('bannedwebsite.com')) . "<br />\r\n";
    echo ban_domain($text2, array('bannedwebsite.com')) . "<br />\r\n";
    echo ban_domain($text3, array('bannedwebsite.com')) . "<br />\r\n";
   
    ?>

 

by: soapergemPosted on 2006-12-20 at 17:30:41ID: 18177848

Which will print out the following:

Hey everybody, go to <spam link removed> for great deals on ...
Hey everybody, go to <spam link removed>
<spam link removed>

 

by: TerryAtOpusPosted on 2006-12-20 at 18:23:04ID: 18177958

Won't that be extremely inefficient if there's a large list of banned urls? (which there quite likely will be)

Instead, I'd guess you need something to find the domain names in the text, and compare just the domain names against your list of banned ones. Then if you find a match, do a substitute.

 

by: soapergemPosted on 2006-12-20 at 18:43:24ID: 18178015

Honestly I don't think it would be all that bad. It's not like I'm using a for-each loop or anything; the regex engine gets to do all the searching in one fell swoop. Your proposal of searching *only* for the domains would work fine, but it wouldn't look as pretty. You'd have extra bits lying around like "[url=" and "][/url]". True, you could go around for a second pass to get rid of those, but that's costly as far as efficiency is concerned as well. Plus I doubt that the cost would be all that great to begin with--probably a couple hundred nanoseconds. No big loss.

 

by: jk2001Posted on 2006-12-21 at 14:14:57ID: 18183724

What I've done is count matches against a list of spammy strings and urls, and if there are two or more matches, block the post completely.  If that's too restrictive, increase the threshold.

You can also do things like count the overall message size, find repetition in the message, and keywords like "viagra" or "online casino".

I know it seems draconian, but spam posts tend to have a lot of URLs, little content, and the same strings.  This technique gets rid of half of them in one motion.

Also, you can test your algorithm by running it against the existing messages to see if it turns up false positives.

 

by: TomeeboyPosted on 2006-12-21 at 14:46:11ID: 18183910

Exactly what I needed soapergem, thanks for the quick solution.  It's implemented and works like a charm ;)

By the way, would there be a quick method of telling if this filter picked up matches?  I'd like to trigger an action within the script in the event that a banned URL has been filtered.  I believe they added a "count" feature in PHP 5+, but unfortunately I'm using version 4.x.

 

by: soapergemPosted on 2006-12-21 at 15:08:40ID: 18184099

Sure. Just before you do the preg_replace(), just do this:

    if ( preg_match($search, $text) )
    {
        //  do something fancy
    }

 

by: TomeeboyPosted on 2006-12-21 at 15:25:34ID: 18184234

I suppose I might as well put the preg_replace inside of that.  That way, I will still be limiting it to one preg function per post for everything that does not trigger the filter.

Thanks again soapergem ;)

20120131-EE-VQP-002

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