search and replace but only if between two markers php regex

I want to replace the <br> tags with a space in a string containing html, but only if the tag is within a <div class="nobr"> ..... </div> sequence. I presume the best way is to use preg_replace but I'm having difficulty finding the regex expression to use for the pattern... (there will be multiple line break tags and more than one "nobr" div)

e.g Simple example - From this:

keep this<br>line break<div class="nobr"> but replace this linebreak<br>with a space</div>

To become:

keep this<br>line break<div class="nobr"> but replace this linebreak with a space</div>

You could first match the nobr elements and use a callback function with preg_replace_callback to replace the <br>.

$pattern = '/<div class="nobr">(.*?)<\/div>/s';
// This regex pattern will match any string that starts with 
// "<div class="nobr">" and ends with "</div>", and capture 
// all characters in between, including line breaks (using the "s" flag).

$string = preg_replace_callback($pattern, function ($matches) {
  return str_replace('<br>', ' ', $matches[0]);
  // The preg_replace_callback function will replace all 
  // "<br>" tags with spaces within each matched "nobr" div.
}, $string);

This would only work if you don't have nested divs, since the closing tag would match the nested div closing tag. If that's the case you need to use a DOM parser:

$doc = new DOMDocument();
$doc->loadHTML($html, LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD);

$xpath = new DOMXPath($doc);
$nobr_divs = $xpath->query("//div[contains(concat(' ', normalize-space(@class), ' '), ' nobr ')]");

foreach ($nobr_divs as $nobr_div) {
    $br_tags = $nobr_div->getElementsByTagName('br');
    foreach ($br_tags as $br_tag) {
        $text = $doc->createTextNode(' ');
        $nobr_div->replaceChild($text, $br_tag);
    }
}

$new_html = $doc->saveHTML();
echo $new_html;