Of course my client doesn't want to put in span class="foo" every time. This is what it should look like:

Therefor I have to write a function. First I have to search for the string "by *FOO" and wrap a span around "by" as well as around "*FOO", since it is also displayed without "by". If I added it to the first function "*FOO" which already has a span would get an additional span. Here is the code:

function colorize_FOO($content) {
	$str = "*FOO";
	$newstr = "<span class=\"foo\">*FOO</span>";
	$html = str_replace($str, $newstr, $content);
	return $html;
}
function colorize_by($content) {
	$str = "by *FOO";
	$newstr = "<span class=\"by\">by</span> *FOO";
	$html = str_replace($str, $newstr, $content);
	return $html;
}

add_filter('the_title', 'colorize_by',1);
add_filter('the_content', 'colorize_by',1);
add_filter('the_title', 'colorize_FOO',1);
add_filter('the_content', 'colorize_FOO',1);

Sure there are more elegant methods, but a better one didn't come in my mind so quickly