select non-breaking spaces for formatting with CSS

Hi,

I’ve now searched for long time, hoping to find a solution.

Sometimes there are non-breaking spaces ( ) in content that users paste to the editor.

I would like to highlight these non-breaking spaces for logged in users or in the backend.
For this it would be necessary to add a class.

What I found is wordpress.org/support/topic/can-you-select-non-breaking-spaces-in-css.

This solution works like a charme for <p> </p> even when I change it to

function user_content_replace ( $content ) {
	return str_replace('&nbsp;', '<span class="highlight">&nbsp;</span>', $content);
}
add_filter ('the_content', 'user_content_replace', 99);

But it doesn’t work at all in nonempty paragraphs as e.g. <p>test test</p>.

Like this a solution would be very appreciated!

Thank you in Advance
Bernhard