preg_replace when pattern NOT followed

Question

Using preg_replace, I would like to replace all instances of a "pipe not followed by http://", If the pipe is followed by http://, then do nothing. For example,

The following string:

http://www.xyz.org/docs/pdfs/2014/file_name_1.pdf|file_name_2.pdf|http://www.xyz.org/docs/pdfs/2014/file_name_3.pdf|

Once run thru preg_replace will become:

http://www.xyz.org/docs/pdfs/2014/file_name_1.pdf|http://www.xyz.org/docs/pdfs/2014/file_name_2.pdf|http://www.xyz.org/docs/pdfs/2014/file_name_3.pdf|

The code I have in place so far replaces all pipes.

$string = trim(preg_replace("/\|\|+/", "|", $string));
$string = str_replace("|", "|http://www.xyz.org/docs/pdfs/2014/", $string);

[1]: stackoverflow.com/questions/3173087/… [2]: stackoverflow.com/questions/19848516/… [3]: stackoverflow.com/questions/10941490/… check these links — sedigo
– sedigo, Commented Jan 21, 2014 at 13:26

Gavin Jackson · Accepted Answer · 2014-01-21 14:52:42Z

2

I think you simply need to use a 'negative lookahead'...

CODE:

<?php
   $a = 'http://www.xyz.org/docs/pdfs/2014/file_name_1.pdf|file_name_2.pdf|http://www.xyz.org/docs/pdfs/2014/file_name_3.pdf|';
   $b = preg_replace("/\|(?!http:\/\/)/",'|http://www.xyz.org/docs/pdfs/2014/',$a);
   echo 'A: '.$a."\n";
   echo 'B: '.$b."\n";
?>

This regular expression \|(?!http:\/\/) will match...

\| a vertical bar
(?!http:\/\/) not followed by http:// (this is the negative lookahead zero width assertion)

OUTPUT:

> A: http://www.xyz.org/docs/pdfs/2014/file_name_1.pdf|file_name_2.pdf|http://www.xyz.org/docs/pdfs/2014/file_name_3.pdf|
> B: http://www.xyz.org/docs/pdfs/2014/file_name_1.pdf|http://www.xyz.org/docs/pdfs/2014/file_name_2.pdf|http://www.xyz.org/docs/pdfs/2014/file_name_3.pdf|http://www.xyz.org/docs/pdfs/2014/

This is a simplified example which also replaces the last trailing '|' in your string, but you get the idea of the regex.
If needed you could handle the trailing bar with this variation of the regex /\|(?!(http:\/\/|$))/ which uses the lookahead to check for end of string too.

edited Jan 21, 2014 at 14:52

answered Jan 21, 2014 at 13:52

Gavin Jackson

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4 Comments

hfrancke Over a year ago

Thanks. Your example removes file_name_2.pdf from the string altogether, where as I need 'xyz.org/docs/pdfs/2014' inserted before the file name.

Gavin Jackson Over a year ago

I realised that and was correcting my reply as you posted :-)

hfrancke Over a year ago

Thanks. I tested yours and it works properly. The remaining issue is that it converts the last pipe with the URL, would prefer the last pipe be stripped from the string. I have tried to make that work but I keep messing up the other aspects.

Gavin Jackson Over a year ago

Doesn't the alternative regex given in the last sentence handle the trailing pipe? /\|(?!(http:\/\/|$))/ I think it should leave the pipe unreplaced as per the desired result you gave in the question.

Mario · Accepted Answer · 2014-01-21 13:34:49Z

0

Maybe by using explode and test one by one

$string = 'http://www.xyz.org/docs/pdfs/2014/file_name_1.pdf|file_name_2.pdf|http://www.xyz.org/docs/pdfs/2014/file_name_3.pdf|';
$string = addHttp($string); 

function addHttp($string) {
    $a_string = explode('|', $string);
    $return_string = '';
    foreach($a_string as $a_s) {
        if(trim($a_s) == '')
            continue; 

        if(preg_match('#^http://#', $a_s))
            $return_string .= $a_s.'|';
        else
            $return_string .= 'http://'.$a_s.'|';
    }

    return $return_string;
}

EDIT : I changed strpos by preg_match

edited Jan 21, 2014 at 13:34

answered Jan 21, 2014 at 13:29

Mario

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2 Comments

hfrancke Over a year ago

I did not think of using an explode. Thank you, that works and I am progressing nicely.

Alan Moore Over a year ago

This works, but you're doing a lot of unnecessary work. preg_replace() is perfect for this job, as Gavin's answer demonstrates.

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