Replace newlines with BR tags, but only inside PRE tags

Question

In stock PHP5, what is a good preg_replace expression for making this transformation:

replace newlines with
, but only

Answer 1

Something like this?

<?php

$content = "<div><pre class='some class'>1
2
3
</pre>
<pre>line 1
line 2
line 3
</pre>
</div>
";

function getInnerHTML($Node)
{
     $Body = $Node->ownerDocument->documentElement->firstChild->firstChild;
     $Document = new DOMDocument();    
     $Document->appendChild($Document->importNode($Body,true));
     return $Document->saveHTML();
}

$dom = new DOMDocument();
$dom->loadHTML( $content );
$preElements = $dom->getElementsByTagName('pre');

if ( count( $preElements ) ) {
    foreach ( $preElements as $pre ) {
    $value = preg_replace( '/\n|\r\n/', '<br/>', $pre->nodeValue  );
    $pre->nodeValue = $value;
    }

    echo html_entity_decode( getInnerHTML( $dom->documentElement ) );
}

Answer 2

Based on something SilentGhost said (which isn't showing up here for some reason):

<?php
$str = "<div><pre class='some class' >1
2
3
< / pre>
<pre>line 1
line 2
line 3
</pre>
</div>";

$out = "<div><pre class='some class' >1<br />2<br />3<br />< / pre>
<pre>line 1<br />line 2<br />line 3<br /></pre>
</div>";

function protect_newlines($str) {
    // \n -> <br />, but only if it's in a pre block
    // protects newlines from Parser::doBlockLevels()
    /* split on <pre ... /pre>, basically.  probably good enough */
    $str = " ".$str;  // guarantee split will be in even positions
    //$parts = preg_split('/(<pre .*  pre>)/Umsxu',$str,-1,PREG_SPLIT_DELIM_CAPTURE);
    $parts = preg_split("/(< \s* pre .* \/ \s* pre \s* >)/Umsxu",$str,-1,PREG_SPLIT_DELIM_CAPTURE);
    foreach ($parts as $idx=>$part) {
        if ($idx % 2) {
            $parts[$idx] = preg_replace("/\n/", "<br />", $part);
        }
    }
    $str = implode('',$parts);
    /* chop off the first space, that we had added */
    return substr($str,1);
}

assert(protect_newlines($str) === $out);
?>