Regulært udtryk driller i preg_replace

<?php
/**
* city_highlighter(string $text, array $cities)
* $text  : Text to perform highlight on
* $cities : Array of city names to highlight
* @return : $text with $cities highlighted.
*/
function city_highlighter($text, $cities)
{
    // Joining $cities into a REGEX-friendly string
    $cities = implode("|", $cities);
   
    // Creating regex pattern (case insensitive)
    $pattern = "/(\b" . $cities . "\b)/i";
   
    // Transform $cities into:
    $transform = '<b>\\1</b>';
   
    $output = preg_replace($pattern, $transform, $text);
   
    return $output;
}

$cities = "Aalborg, Aarhus, Århus, Odense, København";
$cities = explode(", ", $cities);
$text = "I Aalborg findes Gigantium, i Århus findes Musikhuset, i Odense har de hat og briller og i København har de Cirkusbygningen!";

echo '<b>Normal unformatted version of $text:</b><br>' . $text . '<br><br><b>City higlighted version of $text:</b><br>' . city_highlighter($text, $cities);
?>

<b>Normal unformatted version of $text:</b><br>
I Aalborg findes Gigantium, i Århus findes Musikhuset, i Odense har de hat og briller og i København har de Cirkusbygningen!<br><br>

<b>City higlighted version of $text:</b><br>
I <b>Aalborg</b> findes Gigantium, i <b>Århus</b> findes Musikhuset, i <b>Odense</b> har de hat og briller og i <b>København</b> har de Cirkusbygningen!

<?php
/**
* city_highlighter(string $text, array $cities)
* $text  : Text to perform highlight on
* $cities : Array of city names to highlight
* @return : $text with $cities highlighted.
*/
function city_highlighter($text, $cities)
{
    $output = $text;
    foreach($cities as $city)
    {
        // Creating regex pattern (case insensitive)
        $pattern = "/(\b" . $city . "\b)/";
       
        // Transform $cities into:
        $transform = '<b>\\1</b>';
       
        $output = preg_replace($pattern, $transform, $output);
    }
   
    return $output;
}

$cities = "Aalborg, Aarhus, Århus, Ans";
$cities = explode(", ", $cities);
$text = "I Aalborg findes Gigantium, i Århus findes Musikhuset, Ansøgninger bedes sendes til vores afdeling i Ans.";

echo '<b>Normal unformatted version of $text:</b><br>' . $text . '<br><br><b>City higlighted version of $text:</b><br>' . city_highlighter($text, $cities);
?>

<b>Normal unformatted version of $text:</b><br>
I Aalborg findes Gigantium, i Århus findes Musikhuset, Ansøgninger bedes sendes til vores afdeling i Ans.<br<br>

<b>City higlighted version of $text:</b><br>
I <b>Aalborg</b> findes Gigantium, i <b>Århus</b> findes Musikhuset, Ansøgninger bedes sendes til vores afdeling i <b>Ans</b>.