PCRE - Negative Lookbehind Assertion problem

[Jim]

Hi,

I'm trying to prefix the "src" attribute of all "img" elements with a
given string, $prefix. Here's what I've got:

preg_replace('/\<img(.+?)src="(?<!=http)(.+?)"(.+?)\/>/', '<img
$1src="' . $prefix . '$2"$3/>', $content);

The problem comes in that it always performs the replace, even when
there's an "http" in the source attribute.

Here's the source for a quick test if anyone's interested:

<?php

$prefix = 'http://mydomain.com/';

$content = <<<EOT
<img src="images/test.gif" alt="Test Image" />
<img src="http://shouldnt.be.changed/images/test.gif" alt="Test
Image" />
EOT;

echo preg_replace('/\<img(.+?)src="(?<!=http)(.+?)"(.+?)\/>/', '<img
$1src="' . $prefix . '$2"$3/>', $content);

/*
* First img element's src attribute should be changed, but not the
second's.
*/

?>

Thanks to all,

Jim.

[Jim]

I just spotted one error, the equals sign after the negative
lookbehind, although that seems to have made little difference.
Updated code follows:

<?php

$prefix = 'http://mydomain.com/';

$content = <<<EOT
<img src="images/test.gif" alt="Test Image" />
<img src="http://shouldnt.be.changed/images/test.gif" alt="Test
Image" />
EOT;

echo preg_replace('/\<img(.+?)src="(?<!http)(.+?)"(.+?)\/>/', '<img
$1src="' . $prefix . '$2"$3/>', $content);

/*
* First img element's src attribute should be
* changed, but not the second's.
*/

?>

[Rik Wasmus]

On Mon, 29 Oct 2007 22:05:41 +0100, Jim <ji***@yahoo.comwrote:

I just spotted one error, the equals sign after the negative
lookbehind, although that seems to have made little difference.
Updated code follows:

<?php

$prefix = 'http://mydomain.com/';

$content = <<<EOT
<img src="images/test.gif" alt="Test Image" />
<img src="http://shouldnt.be.changed/images/test.gif" alt="Test
Image" />
EOT;

echo preg_replace('/\<img(.+?)src="(?<!http)(.+?)"(.+?)\/>/', '<img
$1src="' . $prefix . '$2"$3/>', $content);

/*
* First img element's src attribute should be
* changed, but not the second's.
*/

?>

Well, taking a guess here, but 'http://' is not preceded by 'http', so it
will match in $2. Lookaheads/behinds without a literal match is often not
a very nice way to go. I'd use the preg_replace_callback or the /e
modifier to let a function examine the match and act apropriately.

--
Rik Wasmus

[Jim]

Well, taking a guess here, but 'http://' is not preceded by 'http', so it
will match in $2. Lookaheads/behinds without a literal match is often not
a very nice way to go.

Now I'm really confused, I thought that was the idea of the lookbehind
assertion, oh well.

I'd use the preg_replace_callback or the /e
modifier to let a function examine the match and act apropriately.

I'll give that route a try.

Thanks for your help Rik.

Jim.