How do I programmatically add rel="external" to external links in a string of HTML?

Question

How can I check if links from a string variable are external? This string is the site content (like comments, articles etc).

And if they are, how do I append a external value to their rel attribute? And if they don't have this attribute, append rel="external" ?

score 6 · Accepted Answer · answered Apr 10 '11 at 00:39

A HTML parser is appropriate for input filtering, but for modifying output you'll need the performance of a simpleminded regex solution. In this case a callback regex would do:

$html = preg_replace_callback("#<a\s[^>]*href="(http://[^"]+)"[^>]*>#",
     "cb_ext_url", $html);

function cb_ext_url($match) {
    list ($orig, $url) = $match;
    if (strstr($url, "http://localhost/")) {
        return $orig;
    }
    elseif (strstr($orig, "rel=")) {
        return $orig;
    }
    else {
        return rtrim($orig, ">") . ' rel="external">';
    }
}

You'll probably need more fine-grained checks. But that's the general approach.

Very clean, accurate and efficient solution. +1 – ridgerunner Apr 10 '11 at 00:59 — ridgerunner, Apr 10 '11 at 00:59

score 2 · Answer 2 · edited May 23 '17 at 11:55

2

Use an XML parser, like SimpleXML. Regex isn't made to do XML/HTML parsing, and here's a perfect explanation of what happens when you do: RegEx match open tags except XHTML self-contained tags.

Parse the input as XML, use the parser to select the required elements, edit their properties using the parser, and spit them back out.

It'll save you a headache, as regex makes me cry...

Here's my way of doing this (didn't test it):

<?php

$xmlString = "This is where the HTML of your site should go. Make sure it's valid!";

$xml = new SimpleXMLElement($xmlString);

foreach($xml->getElementsByTagName('a') as $a)
{
  $attributes = $a->attributes();

  if (isThisExternal($attributes['href']))
  {
    $a['rel'] = 'external';
  }
}

echo $xml->asXml();

?>

edited May 23 '17 at 11:55

Community

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answered Apr 10 '11 at 00:25

Blender

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but wouldn't this be slower than using regex? where can I find a example of xhtml parsing with simplexml? – Alex Apr 10 '11 at 00:26
@Alex, maybe, but better. - @Blender, ..and just pray that the page is valid XML.. – halfdan Apr 10 '11 at 00:29
Regex makes your brains explode out of your ears, as it's basically impossible to do anything like this with regex. If you want to see the level of complexity it takes to validate a *single* email address with regex, here's something to look at: http://www.ex-parrot.com/pdw/Mail-RFC822-Address.html – Blender Apr 10 '11 at 00:29
2

@Blender if Alex is just parsing the content looking for `` tags, wouldn't regex be appropriate? Not as a general-purpose parser, hell no, but just looking for anchors... – Farray Apr 10 '11 at 00:30
It'll get ugly, but if you want, try it. – Blender Apr 10 '11 at 00:32
@Blender: And it's still useless, as there are many working email addresses out there in the wild that do not conform to RFC822. – Lightness Races in Orbit Apr 10 '11 at 00:35
If it's that hard to (attempt to) parse an email, imagine a regex HTML parser. THE HORROR! – Blender Apr 10 '11 at 00:37
@Alex, if you'd like, try to time the speed differences. I doubt that they would be noticeable. – Blender Apr 10 '11 at 00:44
@Blender Ironic that the line with "Make sure it's valid!" isn't properly escaped, eh? ;) – Farray Apr 10 '11 at 00:48
What? Where? I see nothing invalid ;) – Blender Apr 10 '11 at 00:50

Jimmy Sawczuk · Answer 3 · 2011-04-10T00:38:38.387

It might be easier to do something like this on the client side, using jQuery:

<script type="text/javascript">
    $(document).ready(function()
    {
        $.each($('a'), function(idx, tag)
        {
            // you might make this smarter and throw out URLS like 
            // http://www.otherdomain.com/yourdomain.com
            if ($(tag).attr('href').indexOf('yourdomain.com') < 0)
            {
                $(tag).attr('rel', 'external');
            }
        });
    });
</script>

As Craig White points out though, this doesn't do anything SEO-wise and won't help users who have JavaScript disabled.

Good idea. Not right though for Google or users with JS disabled. — Craig White, Apr 10 '11 at 00:35

How do I programmatically add rel="external" to external links in a string of HTML?

3 Answers3

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