How do you write a Wordpress function to put a Span around the first word of the Title?

Question

I want to replace first word in title to have <span></span> inside.

Example for Wordpress title

<h2 class="entry-title"><a href="#">Welcome to Wordpress</a></h2>

I want to be like this

<h2 class="entry-title"><a href="#"><span>Welcome</span> to Wordpress</a></h2>

the function

function span_on_title($span) {
 return preg_replace('', '', $span, 1);
}
add_filter('the_title','span_on_title');

May i know what to put on the preg_replace

Answer 1

  $title = '<h2 class="entry-title"><a href="#">Welcome to Wordpress</a></h2>';

  $title = preg_replace('/<a([^>]+)>([a-zA-Z]+)\s/i', '<a$1><span>$2</span> ', $title);

  return $title;