PHP-REGEX - 在pre标签内用<br>替换换行符

Question

I have

<pre>
Line one
Line two
Line three
Line four
Line five
Line six
</pre>

If I strip the pre tags, it becomes

Line one Line two Line three Line four Line five Line six

What would be the regex for replacing new lines with a br so that after stripping pre tag each line is separate.

Answer 1

At each position you need to check whether or not you are inside a valid <pre> tag:

~(?s)(?<!<pre>)\R(?!</pre>)(?=((?!<pre>).)*</pre>)~

Explanation:

(?s)                # Set DOT_ALL modifier
(?<!<pre>)          # Assert if we are not immediately after an opening <pre> tag
\R                  # We need new-lines only
(?!</pre>)          # Not followed by a closing </pre> tag
(?=                 # Beginning of a positive lookahead
    ((?!<pre>).)*   # To look if we are not behind an opening <pre> tag (inside a <pre> tag)
    </pre>          # Which has a closing </pre> tag
)                   # End of lookahead

Live demo

Note: It doesn't provide expected results if you have nested <pre> tags (!)

But if you are comfortable to work with DOM then there is a more suitable solution for this:

<?php

$html = <<< HTML
<div>
<div>
test
test
test
</div>
<pre>
Line one
Line two
Line three
Line four
Line five
Line six
</pre>
</div>
HTML;

$dom = new DOMDocument;
@$dom->loadHTML($html, LIBXML_HTML_NOIMPLIED  | LIBXML_HTML_NODEFDTD);
$preTags = $dom->getElementsByTagName('pre');

foreach ($preTags as $key => $pre) {
    $pre->nodeValue = str_replace(PHP_EOL, '~*~*', $pre->nodeValue);
}

echo str_replace("~*~*", '<br />', $dom->saveHTML());

Output:

<div>
<div>
test
test
test
</div>
<pre><br />Line one<br />Line two<br />Line three<br />Line four<br />Line five<br />Line six<br /></pre>
</div>