regex to turn URLs into links without messing with existing links in the text

Question

Hi there I am trying to convert URLs in a piece of text into hyperlinks - using regular expressions. I have managed to achieve this but the problem is when there are already existing links in the text

so

bla bla blah www.google.com bla blah <a href="www.google.com">www.google.com</a>

should result in

bla bla blah <a href="http://www.google.com"&gt;www.google.com&lt;/a&gt; bla blah <a href="www.google.com">www.google.com</a>

not

bla bla blah <a href="http://www.google.com"&gt;www.google.com&lt;/a&gt; bla blah <a href="<a href="http://www.google.com"&gt;www.google.com&lt;/a&gt;&lt;/a&gt;"&gt;&lt;a href="http://www.google.com"&gt;www.google.com&lt;/a&gt;&lt;/a&gt;

Answer 1

This is almost impossible to do with a single regular expression. I would instead recommend a state-machine based approach. Something like this (in pseudo-code)

state = OUTSIDE_LINK
for pos (0 .. length input)
   switch state
   case OUTSIDE_LINK
     if substring at pos matches /<a/
       state = INSIDE_LINK
     else if substring at pos matches /(www.\S+|\S+.com|\S+.org)/
       substitute link
   case INSIDE_LINK
     if substring at post matches /<\/a>/
       state = OUTSIDE_LINK

Answer 2

Another way of doing it (in php)

    $strParts = preg_split( '/(<[^>]+>)/', $html, -1, PREG_SPLIT_DELIM_CAPTURE | PREG_SPLIT_NO_EMPTY );
    foreach( $strParts as $key=>$part ) {

        /*check this part isn't a tag or inside a link*/
        if( !(preg_match( '@(<[^>]+>)@', $part ) || preg_match( '@(<a[^>]+>)@', $strParts[$key - 1] )) ) {
            $strParts[$key] = preg_replace( '@((http(s)?://)?(\S+\.{1}[^\s\,\.\!]+))@', '<a href="http$3://$4">$1</a>', $strParts[$key] );
        }

    }
    $html = implode( $strParts );

Answer 3

Another trick is to guard all the existing links by encoding the code, then replacing urls with links, and then un-encoding the guarded links.

$data = 'test http://foo <a href="http://link"&gt;LINK&lt;/a&gt; test';

$data = preg_replace_callback('/(<a href=".+?<\/a>)/','guard_url',$data);

$data = preg_replace_callback('/(http:\/\/.+?)([ .\\n\\r])/','link_url',$data);

$data = preg_replace_callback('/{{([a-zA-Z0-9+]+?)}}/','unguard_url',$data);

print $data;

function guard_url($arr) { return '{{'.base64_encode($arr[1]).'}}'; }
function unguard_url($arr) { return base64_decode($arr[1]); }
function link_url($arr) { return '<a href="'.$arr[1].'">'.$arr[1].'</a>'.$arr[2]; }

The code above is just a proof of concept, and doesn't handle all situations. Still, you can see that the code is pretty straightforward.

Answer 4

Finally finished it:

function add_url_links($data)
{
        $data = preg_replace_callback('/(<a href=.+?<\/a>)/','guard_url',$data);

        $data = preg_replace_callback('/(http:\/\/.+?)([ \\n\\r])/','link_url',$data);
        $data = preg_replace_callback('/^(http:\/\/.+?)/','link_url',$data);
        $data = preg_replace_callback('/(http:\/\/.+?)$/','link_url',$data);

        $data = preg_replace_callback('/{{([a-zA-Z0-9+=]+?)}}/','unguard_url',$data);

        return $data;
}

function guard_url($arr) { return '{{'.base64_encode($arr[1]).'}}'; }
function unguard_url($arr) { return base64_decode($arr[1]); }
function link_url($arr) { return guard_url(array('','<a href="'.$arr[1].'">'.$arr[1].'</a>')).$arr[2]; }

Answer 5

Why not strip the links first and treat the entire text fairly and start all over and scoop out the links? Stripping sounds easy enough.