Bolding specific word(s)/parts of widget title

Is there a way to bold a specific word or part of a widget title? For example the title “Foo Bar”, “Foo” here would be bold.

I tried doing the following, but the span gets outputted in the title:

add_filter( 'widget_title', 'test', 10, 1);
function test( $content ) {
    $test = explode(' ', $content);

    if ( !empty($test[1] ) ) {
        $title="<span class="this_is_bold">" . $test[0] . '</span>';
        $title = $title . ' ' . $test[1];
    }

    return $title;
}

This bolds the first words (or entire word, if only one) of my widget titles:

add_filter( 'widget_title', function ( $title ) {
    return preg_replace("/^(.+?)( |$)/", "<strong>$0</strong>", $title, 1);
}, 10, 1);

Breaking down the regex pattern, we have:

• ^ is the start of the line
• (.+?) is anything between our next group (in this context, a word)
• ( |$) means either the first space, or the end of the line
  • the first space would be a normal title with multiple words
  • the end of line would mean a space wasn’t found, so the title is just one word

In preg_replace(), we have a fourth attribute set to 1, it’s the limit. This means the replace will only work on the first instance (which doesn’t matter much in this context, as we know $title is always a single line so because of our start of line anchor ^ we know it can only work once always – but hey, why not).

Please keep in mind I put the code in an anonymous function for minimal code (showing off). If this is for a plugin or a public theme, best to stick to doing the function call as you originally did.